diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index ca5d460..48ed355 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -2291,6 +2291,7 @@
   {
     "reviewers": [
       {
+        "input": "john.doe@example.com",
         "approvals": {
           "Verified": " 0",
           "Code-Review": " 0"
@@ -2328,6 +2329,7 @@
 
   )]}'
   {
+    "input": "MyProjectVerifiers",
     "error": "The group My Group has 15 members. Do you want to add them all as reviewers?",
     "confirm": true
   }
@@ -2342,7 +2344,7 @@
   Content-Type: application/json; charset=UTF-8
 
   {
-    "reviewer": "MyProjectVerifiers",
+    "input": "MyProjectVerifiers",
     "confirmed": true
   }
 ----
@@ -4051,6 +4053,9 @@
 [options="header",cols="1,^1,5"]
 |===========================
 |Field Name    ||Description
+|`input`    ||
+Value of the `reviewer` field from link:#reviewer-input[ReviewerInput]
+set while adding the reviewer.
 |`reviewers`   |optional|
 The newly added reviewers as a list of link:#reviewer-info[
 ReviewerInfo] entities.
diff --git a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/change/ChangeReviewersIT.java b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/change/ChangeReviewersIT.java
new file mode 100644
index 0000000..0bd70ef
--- /dev/null
+++ b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/change/ChangeReviewersIT.java
@@ -0,0 +1,110 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.acceptance.rest.change;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import com.google.gerrit.acceptance.AbstractDaemonTest;
+import com.google.gerrit.acceptance.PushOneCommit;
+import com.google.gerrit.acceptance.RestResponse;
+import com.google.gerrit.acceptance.TestAccount;
+import com.google.gerrit.extensions.api.changes.AddReviewerInput;
+import com.google.gerrit.extensions.api.changes.AddReviewerResult;
+import com.google.gerrit.server.change.PostReviewers;
+import com.google.gson.stream.JsonReader;
+
+import org.junit.Test;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class ChangeReviewersIT extends AbstractDaemonTest {
+  @Test
+  public void addGroupAsReviewer() throws Exception {
+    // Set up two groups, one that is too large too add as reviewer, and one
+    // that is too large to add without confirmation.
+    String largeGroup = createGroup("largeGroup");
+    String mediumGroup = createGroup("mediumGroup");
+
+    int largeGroupSize = PostReviewers.DEFAULT_MAX_REVIEWERS + 1;
+    int mediumGroupSize = PostReviewers.DEFAULT_MAX_REVIEWERS_WITHOUT_CHECK + 1;
+    List<TestAccount> users = new ArrayList<>(largeGroupSize);
+    List<String> largeGroupUsernames = new ArrayList<>(mediumGroupSize);
+    List<String> mediumGroupUsernames = new ArrayList<>(mediumGroupSize);
+    for (int i = 0; i < largeGroupSize; i++) {
+      users.add(accounts.create(name("u" + i), name("u" + i + "@example.com"),
+          "Full Name " + i));
+      largeGroupUsernames.add(users.get(i).username);
+      if (i < mediumGroupSize) {
+        mediumGroupUsernames.add(users.get(i).username);
+      }
+    }
+    gApi.groups().id(largeGroup).addMembers(
+        largeGroupUsernames.toArray(new String[largeGroupSize]));
+    gApi.groups().id(mediumGroup).addMembers(
+        mediumGroupUsernames.toArray(new String[mediumGroupSize]));
+
+    // Attempt to add overly large group as reviewers.
+    PushOneCommit.Result r = createChange();
+    String changeId = r.getChangeId();
+    AddReviewerResult result = addReviewer(changeId, largeGroup);
+    assertThat(result.input).isEqualTo(largeGroup);
+    assertThat(result.confirm).isNull();
+    assertThat(result.error)
+        .contains("has too many members to add them all as reviewers");
+    assertThat(result.reviewers).isNull();
+
+    // Attempt to add medium group without confirmation.
+    result = addReviewer(changeId, mediumGroup);
+    assertThat(result.input).isEqualTo(mediumGroup);
+    assertThat(result.confirm).isTrue();
+    assertThat(result.error)
+        .contains("has " + mediumGroupSize + " members. Do you want to add them"
+            + " all as reviewers?");
+    assertThat(result.reviewers).isNull();
+
+    // Add medium group with confirmation.
+    AddReviewerInput in = new AddReviewerInput();
+    in.reviewer = mediumGroup;
+    in.confirmed = true;
+    result = addReviewer(changeId, in);
+    assertThat(result.input).isEqualTo(mediumGroup);
+    assertThat(result.confirm).isNull();
+    assertThat(result.error).isNull();
+    assertThat(result.reviewers).hasSize(mediumGroupSize);
+  }
+
+  private AddReviewerResult addReviewer(String changeId, String reviewer)
+      throws Exception {
+    AddReviewerInput in = new AddReviewerInput();
+    in.reviewer = reviewer;
+    return addReviewer(changeId, in);
+  }
+
+  private AddReviewerResult addReviewer(String changeId, AddReviewerInput in)
+      throws Exception {
+    RestResponse resp =
+        adminRestSession.post("/changes/" + changeId + "/reviewers", in);
+    return readContentFromJson(resp, AddReviewerResult.class);
+  }
+
+  private static <T> T readContentFromJson(RestResponse r, Class<T> clazz)
+      throws Exception {
+    r.assertOK();
+    JsonReader jsonReader = new JsonReader(r.getReader());
+    jsonReader.setLenient(true);
+    return newGson().fromJson(jsonReader, clazz);
+  }
+}
\ No newline at end of file
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/AddReviewerResult.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/AddReviewerResult.java
new file mode 100644
index 0000000..3bd93a4
--- /dev/null
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/AddReviewerResult.java
@@ -0,0 +1,80 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.api.changes;
+
+import com.google.gerrit.common.Nullable;
+
+import java.util.List;
+
+/**
+ * Result object representing the outcome of a request to add a reviewer.
+ */
+public class AddReviewerResult {
+  /**
+   * The identifier of an account or group that was to be added as a reviewer.
+   */
+  public String input;
+
+  /**
+   * If non-null, a string describing why the reviewer could not be added.
+   */
+  @Nullable
+  public String error;
+
+  /**
+   * Non-null and true if the reviewer cannot be added without explicit
+   * confirmation. This may be the case for groups of a certain size.
+   */
+  @Nullable
+  public Boolean confirm;
+
+  /**
+   * @{List} of individual reviewers added to the change. The size of this list
+   * may be greater than one (e.g. when a group is added). Null if no reviewers
+   * were added.
+   */
+  @Nullable
+  public List<ReviewerInfo> reviewers;
+
+  /**
+   * Constructs a partially initialized result for the given reviewer.
+   *
+   * @param input String identifier of an account or group, from user request
+   */
+  public AddReviewerResult(String input) {
+    this.input = input;
+  }
+
+  /**
+   * Constructs an error result for the given account.
+   *
+   * @param reviewer String identifier of an account or group
+   * @param error Error message
+   */
+  public AddReviewerResult(String reviewer, String error) {
+    this(reviewer);
+    this.error = error;
+  }
+
+  /**
+   * Constructs a needs-confirmation result for the given account.
+   *
+   * @param confirm Whether confirmation is needed.
+   */
+  public AddReviewerResult(String reviewer, boolean confirm) {
+    this(reviewer);
+    this.confirm = confirm;
+  }
+}
\ No newline at end of file
diff --git a/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/ReviewerInfo.java b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/ReviewerInfo.java
new file mode 100644
index 0000000..c81f8aa
--- /dev/null
+++ b/gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/changes/ReviewerInfo.java
@@ -0,0 +1,41 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.extensions.api.changes;
+
+import com.google.gerrit.common.Nullable;
+import com.google.gerrit.extensions.common.AccountInfo;
+
+import java.util.Map;
+
+/**
+ * Account and approval details for an added reviewer.
+ */
+public class ReviewerInfo extends AccountInfo {
+  /**
+   * {@link Map} of label name to initial value for each approval the reviewer
+   * is responsible for.
+   */
+  @Nullable
+  public Map<String, String> approvals;
+
+  public ReviewerInfo(Integer id) {
+    super(id);
+  }
+
+  @Override
+  public String toString() {
+    return username;
+  }
+}
\ No newline at end of file
diff --git a/gerrit-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/HighlightSuggestOracle.java b/gerrit-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/HighlightSuggestOracle.java
index 06245c6..f4ff9ea4 100644
--- a/gerrit-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/HighlightSuggestOracle.java
+++ b/gerrit-gwtexpui/src/main/java/com/google/gwtexpui/safehtml/client/HighlightSuggestOracle.java
@@ -115,7 +115,8 @@
         @Override
         public int compare(String s1, String s2) {
           return Integer.compare(s2.length(), s1.length());
-        }});
+        }
+      });
 
       List<String> result = new ArrayList<>();
       for (String s : queryTerms) {
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/GetReviewer.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/GetReviewer.java
index c90b3bc..533468d 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/GetReviewer.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/GetReviewer.java
@@ -14,8 +14,8 @@
 
 package com.google.gerrit.server.change;
 
+import com.google.gerrit.extensions.api.changes.ReviewerInfo;
 import com.google.gerrit.extensions.restapi.RestReadView;
-import com.google.gerrit.server.change.ReviewerJson.ReviewerInfo;
 import com.google.gwtorm.server.OrmException;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/ListReviewers.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/ListReviewers.java
index 9a0c691..ccbd552 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/ListReviewers.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/ListReviewers.java
@@ -14,11 +14,11 @@
 
 package com.google.gerrit.server.change;
 
+import com.google.gerrit.extensions.api.changes.ReviewerInfo;
 import com.google.gerrit.extensions.restapi.RestReadView;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.server.ReviewDb;
 import com.google.gerrit.server.ApprovalsUtil;
-import com.google.gerrit.server.change.ReviewerJson.ReviewerInfo;
 import com.google.gwtorm.server.OrmException;
 import com.google.inject.Inject;
 import com.google.inject.Provider;
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java
index 432ae4d..5090f8a 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PatchSetInserter.java
@@ -72,7 +72,6 @@
 
   // Injected fields.
   private final PatchSetInfoFactory patchSetInfoFactory;
-  private final ReviewDb db;
   private final CommitValidators.Factory commitValidatorsFactory;
   private final ReplacePatchSetSender.Factory replacePatchSetFactory;
   private final RevisionCreated revisionCreated;
@@ -109,8 +108,7 @@
   private ReviewerSet oldReviewers;
 
   @AssistedInject
-  public PatchSetInserter(ReviewDb db,
-      ApprovalsUtil approvalsUtil,
+  public PatchSetInserter(ApprovalsUtil approvalsUtil,
       ApprovalCopier approvalCopier,
       ChangeMessagesUtil cmUtil,
       PatchSetInfoFactory patchSetInfoFactory,
@@ -121,7 +119,6 @@
       @Assisted ChangeControl ctl,
       @Assisted PatchSet.Id psId,
       @Assisted RevCommit commit) {
-    this.db = db;
     this.approvalsUtil = approvalsUtil;
     this.approvalCopier = approvalCopier;
     this.cmUtil = cmUtil;
@@ -208,6 +205,7 @@
   @Override
   public boolean updateChange(ChangeContext ctx)
       throws ResourceConflictException, OrmException, IOException {
+    ReviewDb db = ctx.getDb();
     ChangeControl ctl = ctx.getControl();
 
     change = ctx.getChange();
@@ -222,12 +220,12 @@
 
     List<String> newGroups = groups;
     if (newGroups.isEmpty()) {
-      PatchSet prevPs = psUtil.current(ctx.getDb(), ctx.getNotes());
+      PatchSet prevPs = psUtil.current(db, ctx.getNotes());
       if (prevPs != null) {
         newGroups = prevPs.getGroups();
       }
     }
-    patchSet = psUtil.insert(ctx.getDb(), ctx.getRevWalk(), ctx.getUpdate(psId),
+    patchSet = psUtil.insert(db, ctx.getRevWalk(), ctx.getUpdate(psId),
         psId, commit, draft, newGroups, null);
 
     if (sendMail) {
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java
index fabedd7..4b15fce 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/PostReviewers.java
@@ -21,6 +21,8 @@
 import com.google.gerrit.common.data.GroupDescription;
 import com.google.gerrit.common.errors.NoSuchGroupException;
 import com.google.gerrit.extensions.api.changes.AddReviewerInput;
+import com.google.gerrit.extensions.api.changes.AddReviewerResult;
+import com.google.gerrit.extensions.api.changes.ReviewerInfo;
 import com.google.gerrit.extensions.restapi.BadRequestException;
 import com.google.gerrit.extensions.restapi.RestApiException;
 import com.google.gerrit.extensions.restapi.RestModifyView;
@@ -38,8 +40,6 @@
 import com.google.gerrit.server.account.AccountLoader;
 import com.google.gerrit.server.account.AccountsCollection;
 import com.google.gerrit.server.account.GroupMembers;
-import com.google.gerrit.server.change.ReviewerJson.PostResult;
-import com.google.gerrit.server.change.ReviewerJson.ReviewerInfo;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.extensions.events.ReviewerAdded;
 import com.google.gerrit.server.git.BatchUpdate;
@@ -128,7 +128,7 @@
   }
 
   @Override
-  public PostResult apply(ChangeResource rsrc, AddReviewerInput input)
+  public AddReviewerResult apply(ChangeResource rsrc, AddReviewerInput input)
       throws UpdateException, OrmException, RestApiException, IOException {
     if (input.reviewer == null) {
       throw new BadRequestException("missing reviewer field");
@@ -136,7 +136,7 @@
 
     try {
       Account.Id accountId = accounts.parse(input.reviewer).getAccountId();
-      return putAccount(reviewerFactory.create(rsrc, accountId));
+      return putAccount(input.reviewer, reviewerFactory.create(rsrc, accountId));
     } catch (UnprocessableEntityException e) {
       try {
         return putGroup(rsrc, input);
@@ -148,11 +148,11 @@
     }
   }
 
-  private PostResult putAccount(ReviewerResource rsrc)
+  private AddReviewerResult putAccount(String reviewer, ReviewerResource rsrc)
       throws OrmException, UpdateException, RestApiException {
     Account member = rsrc.getReviewerUser().getAccount();
     ChangeControl control = rsrc.getReviewerControl();
-    PostResult result = new PostResult();
+    AddReviewerResult result = new AddReviewerResult(reviewer);
     if (isValidReviewer(member, control)) {
       addReviewers(rsrc.getChangeResource(), result,
           ImmutableMap.of(member.getId(), control));
@@ -160,10 +160,10 @@
     return result;
   }
 
-  private PostResult putGroup(ChangeResource rsrc, AddReviewerInput input)
+  private AddReviewerResult putGroup(ChangeResource rsrc, AddReviewerInput input)
       throws UpdateException, RestApiException, OrmException, IOException {
     GroupDescription.Basic group = groupsCollection.parseInternal(input.reviewer);
-    PostResult result = new PostResult();
+    AddReviewerResult result = new AddReviewerResult(input.reviewer);
     if (!isLegalReviewerGroup(group.getGroupUUID())) {
       result.error = MessageFormat.format(
           ChangeMessages.get().groupIsNotAllowed, group.getName());
@@ -227,7 +227,7 @@
 
 
   private void addReviewers(
-      ChangeResource rsrc, PostResult result, Map<Account.Id, ChangeControl> reviewers)
+      ChangeResource rsrc, AddReviewerResult result, Map<Account.Id, ChangeControl> reviewers)
       throws OrmException, RestApiException, UpdateException {
     try (BatchUpdate bu = batchUpdateFactory.create(
             dbProvider.get(), rsrc.getProject(), rsrc.getUser(), TimeUtil.nowTs())) {
@@ -240,8 +240,8 @@
       for (PatchSetApproval psa : op.added) {
         // New reviewers have value 0, don't bother normalizing.
         result.reviewers.add(
-          json.format(new ReviewerInfo(
-              psa.getAccountId()), reviewers.get(psa.getAccountId()),
+          json.format(new ReviewerInfo(psa.getAccountId().get()),
+              reviewers.get(psa.getAccountId()),
               ImmutableList.of(psa)));
       }
 
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java b/gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java
index e24a290..69cd439 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/change/ReviewerJson.java
@@ -23,7 +23,7 @@
 import com.google.gerrit.common.data.Permission;
 import com.google.gerrit.common.data.PermissionRange;
 import com.google.gerrit.common.data.SubmitRecord;
-import com.google.gerrit.extensions.common.AccountInfo;
+import com.google.gerrit.extensions.api.changes.ReviewerInfo;
 import com.google.gerrit.reviewdb.client.Account;
 import com.google.gerrit.reviewdb.client.PatchSet;
 import com.google.gerrit.reviewdb.client.PatchSetApproval;
@@ -40,7 +40,6 @@
 
 import java.util.Collection;
 import java.util.List;
-import java.util.Map;
 import java.util.TreeMap;
 
 @Singleton
@@ -67,7 +66,7 @@
     AccountLoader loader = accountLoaderFactory.create(true);
     for (ReviewerResource rsrc : rsrcs) {
       ReviewerInfo info = format(new ReviewerInfo(
-          rsrc.getReviewerUser().getAccountId()),
+          rsrc.getReviewerUser().getAccountId().get()),
           rsrc.getReviewerControl());
       loader.put(info);
       infos.add(info);
@@ -132,18 +131,4 @@
 
     return out;
   }
-
-  public static class ReviewerInfo extends AccountInfo {
-    Map<String, String> approvals;
-
-    protected ReviewerInfo(Account.Id id) {
-      super(id.get());
-    }
-  }
-
-  public static class PostResult {
-    public List<ReviewerInfo> reviewers;
-    public String error;
-    Boolean confirm;
-  }
 }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/BatchUpdateReviewDb.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/BatchUpdateReviewDb.java
index 8351e41..1de98d3 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/BatchUpdateReviewDb.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/BatchUpdateReviewDb.java
@@ -37,6 +37,18 @@
     return changesWrapper;
   }
 
+  @Override
+  public void commit() {
+    throw new UnsupportedOperationException(
+        "do not call commit; BatchUpdate always manages transactions");
+  }
+
+  @Override
+  public void rollback() {
+    throw new UnsupportedOperationException(
+        "do not call rollback; BatchUpdate always manages transactions");
+  }
+
   private static class BatchUpdateChanges extends ChangeAccessWrapper {
     private BatchUpdateChanges(ChangeAccess delegate) {
       super(delegate);
@@ -51,14 +63,14 @@
     @Override
     public void upsert(Iterable<Change> instances) {
       throw new UnsupportedOperationException(
-          "do not call upsert; either use InsertChangeOp for insertion, or"
-          + " ChangeContext#saveChange() for update");
+          "do not call upsert; existing changes are updated automatically,"
+          + " or use InsertChangeOp for insertion");
     }
 
     @Override
     public void update(Iterable<Change> instances) {
       throw new UnsupportedOperationException(
-          "do not call update; use ChangeContext#saveChange()");
+          "do not call update; change is updated automatically");
     }
 
     @Override
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollection.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollection.java
index cfdedd0..741fee1 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollection.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/GarbageCollection.java
@@ -17,7 +17,6 @@
 import com.google.common.collect.Sets;
 import com.google.gerrit.common.data.GarbageCollectionResult;
 import com.google.gerrit.extensions.events.GarbageCollectorListener;
-import com.google.gerrit.extensions.events.GarbageCollectorListener.Event;
 import com.google.gerrit.extensions.registration.DynamicSet;
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.server.config.GcConfig;
@@ -115,17 +114,10 @@
   }
 
   private void fire(final Project.NameKey p, final Properties statistics) {
-    Event event = new GarbageCollectorListener.Event() {
-      @Override
-      public String getProjectName() {
-        return p.get();
-      }
-
-      @Override
-      public Properties getStatistics() {
-        return statistics;
-      }
-    };
+    if (!listeners.iterator().hasNext()) {
+      return;
+    }
+    Event event = new Event(p, statistics);
     for (GarbageCollectorListener l : listeners) {
       try {
         l.onGarbageCollected(event);
@@ -204,4 +196,24 @@
       writer.print(message);
     }
   }
+
+  private static class Event implements GarbageCollectorListener.Event {
+    private final Project.NameKey p;
+    private final Properties statistics;
+
+    Event(Project.NameKey p, Properties statistics) {
+      this.p = p;
+      this.statistics = statistics;
+    }
+
+    @Override
+    public String getProjectName() {
+      return p.get();
+    }
+
+    @Override
+    public Properties getStatistics() {
+      return statistics;
+    }
+  }
 }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/project/CreateProject.java b/gerrit-server/src/main/java/com/google/gerrit/server/project/CreateProject.java
index 2a447e4..4c77038 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/project/CreateProject.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/project/CreateProject.java
@@ -94,7 +94,7 @@
   private final ProjectJson json;
   private final ProjectControl.GenericFactory projectControlFactory;
   private final GitRepositoryManager repoManager;
-  private final DynamicSet<NewProjectCreatedListener> createdListener;
+  private final DynamicSet<NewProjectCreatedListener> createdListeners;
   private final ProjectCache projectCache;
   private final GroupBackend groupBackend;
   private final ProjectOwnerGroupsProvider.Factory projectOwnerGroups;
@@ -113,7 +113,7 @@
       DynamicSet<ProjectCreationValidationListener> projectCreationValidationListeners,
       ProjectControl.GenericFactory projectControlFactory,
       GitRepositoryManager repoManager,
-      DynamicSet<NewProjectCreatedListener> createdListener,
+      DynamicSet<NewProjectCreatedListener> createdListeners,
       ProjectCache projectCache,
       GroupBackend groupBackend,
       ProjectOwnerGroupsProvider.Factory projectOwnerGroups,
@@ -131,7 +131,7 @@
     this.json = json;
     this.projectControlFactory = projectControlFactory;
     this.repoManager = repoManager;
-    this.createdListener = createdListener;
+    this.createdListeners = createdListeners;
     this.projectCache = projectCache;
     this.groupBackend = groupBackend;
     this.projectOwnerGroups = projectOwnerGroups;
@@ -253,24 +253,7 @@
           createEmptyCommits(repo, nameKey, args.branch);
         }
 
-        NewProjectCreatedListener.Event event = new NewProjectCreatedListener.Event() {
-          @Override
-          public String getProjectName() {
-            return nameKey.get();
-          }
-
-          @Override
-          public String getHeadName() {
-            return head;
-          }
-        };
-        for (NewProjectCreatedListener l : createdListener) {
-          try {
-            l.onNewProjectCreated(event);
-          } catch (RuntimeException e) {
-            log.warn("Failure in NewProjectCreatedListener", e);
-          }
-        }
+        fire(nameKey, head);
 
         return projectCache.get(nameKey).getProject();
       }
@@ -395,4 +378,39 @@
       throw e;
     }
   }
+
+  private void fire(Project.NameKey name, String head) {
+    if (!createdListeners.iterator().hasNext()) {
+      return;
+    }
+
+    Event event = new Event(name, head);
+    for (NewProjectCreatedListener l : createdListeners) {
+      try {
+        l.onNewProjectCreated(event);
+      } catch (RuntimeException e) {
+        log.warn("Failure in NewProjectCreatedListener", e);
+      }
+    }
+  }
+
+  static class Event implements NewProjectCreatedListener.Event {
+    private final Project.NameKey name;
+    private final String head;
+
+    Event(Project.NameKey name, String head) {
+      this.name = name;
+      this.head = head;
+    }
+
+    @Override
+    public String getProjectName() {
+      return name.get();
+    }
+
+    @Override
+    public String getHeadName() {
+      return head;
+    }
+  }
 }
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/project/SetHead.java b/gerrit-server/src/main/java/com/google/gerrit/server/project/SetHead.java
index aa06024..f964311 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/project/SetHead.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/project/SetHead.java
@@ -23,6 +23,7 @@
 import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
 import com.google.gerrit.extensions.restapi.RestModifyView;
 import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
+import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.reviewdb.client.RefNames;
 import com.google.gerrit.server.IdentifiedUser;
 import com.google.gerrit.server.git.GitRepositoryManager;
@@ -53,15 +54,15 @@
 
   private final GitRepositoryManager repoManager;
   private final Provider<IdentifiedUser> identifiedUser;
-  private final DynamicSet<HeadUpdatedListener> headUpdatedListener;
+  private final DynamicSet<HeadUpdatedListener> headUpdatedListeners;
 
   @Inject
   SetHead(GitRepositoryManager repoManager,
       Provider<IdentifiedUser> identifiedUser,
-      DynamicSet<HeadUpdatedListener> headUpdatedListener) {
+      DynamicSet<HeadUpdatedListener> headUpdatedListeners) {
     this.repoManager = repoManager;
     this.identifiedUser = identifiedUser;
-    this.headUpdatedListener = headUpdatedListener;
+    this.headUpdatedListeners = headUpdatedListeners;
   }
 
   @Override
@@ -106,33 +107,52 @@
             throw new IOException("Setting HEAD failed with " + res);
         }
 
-        HeadUpdatedListener.Event event = new HeadUpdatedListener.Event() {
-          @Override
-          public String getProjectName() {
-            return rsrc.getNameKey().get();
-          }
-
-          @Override
-          public String getOldHeadName() {
-            return oldHead;
-          }
-
-          @Override
-          public String getNewHeadName() {
-            return newHead;
-          }
-        };
-        for (HeadUpdatedListener l : headUpdatedListener) {
-          try {
-            l.onHeadUpdated(event);
-          } catch (RuntimeException e) {
-            log.warn("Failure in HeadUpdatedListener", e);
-          }
-        }
+        fire(rsrc.getNameKey(), oldHead, newHead);
       }
       return ref;
     } catch (RepositoryNotFoundException e) {
       throw new ResourceNotFoundException(rsrc.getName());
     }
   }
+
+  private void fire(Project.NameKey nameKey, String oldHead, String newHead) {
+    if (!headUpdatedListeners.iterator().hasNext()) {
+      return;
+    }
+    Event event = new Event(nameKey, oldHead, newHead);
+    for (HeadUpdatedListener l : headUpdatedListeners) {
+      try {
+        l.onHeadUpdated(event);
+      } catch (RuntimeException e) {
+        log.warn("Failure in HeadUpdatedListener", e);
+      }
+    }
+  }
+
+  static class Event implements HeadUpdatedListener.Event {
+    private final Project.NameKey nameKey;
+    private final String oldHead;
+    private final String newHead;
+
+    Event(Project.NameKey nameKey, String oldHead, String newHead) {
+      this.nameKey = nameKey;
+      this.oldHead = oldHead;
+      this.newHead = newHead;
+    }
+
+    @Override
+    public String getProjectName() {
+      return nameKey.get();
+    }
+
+    @Override
+    public String getOldHeadName() {
+      return oldHead;
+    }
+
+    @Override
+    public String getNewHeadName() {
+      return newHead;
+    }
+  }
 }
diff --git a/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java b/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java
index 3208d00..f98d63f 100644
--- a/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java
+++ b/gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryRepositoryManager.java
@@ -83,7 +83,7 @@
       }
     } catch (RepositoryNotFoundException e) {
       repo = new Repo(name);
-      repos.put(name.get().toLowerCase(), repo);
+      repos.put(normalize(name), repo);
     }
     return repo;
   }
@@ -113,12 +113,20 @@
     }
   }
 
+  public synchronized void deleteRepository(Project.NameKey name) {
+    repos.remove(normalize(name));
+  }
+
   private synchronized Repo get(Project.NameKey name)
       throws RepositoryNotFoundException {
-    Repo repo = repos.get(name.get().toLowerCase());
+    Repo repo = repos.get(normalize(name));
     if (repo != null) {
       return repo;
     }
     throw new RepositoryNotFoundException(name.get());
   }
+
+  private static String normalize(Project.NameKey name) {
+    return name.get().toLowerCase();
+  }
 }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html
index c7705a4..40364fa 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.html
@@ -77,10 +77,12 @@
 
           this._clearDiffContent();
 
+          console.time('diff render');
           this.$.processor.process(diff.content).then(function() {
             if (this.isImageDiff) {
               this._builder.renderDiffImages();
             }
+            console.timeEnd('diff render');
             this.fire('render');
           }.bind(this));
         },
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js
index f4fa310..b035af1 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder.js
@@ -17,6 +17,8 @@
   // Prevent redefinition.
   if (window.GrDiffBuilder) { return; }
 
+  var REGEX_ASTRAL_SYMBOL = /[\uD800-\uDBFF][\uDC00-\uDFFF]/;
+
   function GrDiffBuilder(diff, comments, prefs, outputEl) {
     this._diff = diff;
     this._comments = comments;
@@ -30,8 +32,6 @@
   GrDiffBuilder.AMPERSAND_CODE = '&'.charCodeAt(0);
   GrDiffBuilder.SEMICOLON_CODE = ';'.charCodeAt(0);
 
-  GrDiffBuilder.TAB_REGEX = /\t/g;
-
   GrDiffBuilder.LINE_FEED_HTML =
       '<span class="style-scope gr-diff br"></span>';
 
@@ -298,10 +298,10 @@
     td.classList.add(line.type);
     var text = line.text;
     var html = util.escapeHTML(text);
+    html = this._addTabWrappers(html, this._prefs.tab_size);
 
     td.classList.add(line.highlights.length > 0 ?
         'lightHighlight' : 'darkHighlight');
-
     if (line.highlights.length > 0) {
       html = this._addIntralineHighlights(text, html, line.highlights);
     }
@@ -310,7 +310,6 @@
         this._prefs.line_length) {
       html = this._addNewlines(text, html);
     }
-    html = this._addTabWrappers(html);
 
     var contentText = this._createElement('div', 'contentText');
 
@@ -327,12 +326,16 @@
     return td;
   };
 
+  /**
+   * Returns the text length after normalizing unicode and tabs.
+   * @return {Number} The normalized length of the text.
+   */
   GrDiffBuilder.prototype._textLength = function(text, tabSize) {
-    // TODO(andybons): Unicode support.
+    text = text.replace(REGEX_ASTRAL_SYMBOL, '_');
     var numChars = 0;
     for (var i = 0; i < text.length; i++) {
       if (text[i] === '\t') {
-        numChars += tabSize;
+        numChars += tabSize - (numChars % tabSize);
       } else {
         numChars++;
       }
@@ -395,10 +398,34 @@
     return result;
   };
 
-  GrDiffBuilder.prototype._addTabWrappers = function(html) {
-    var htmlStr = this._getTabWrapper(this._prefs.tab_size,
-        this._prefs.show_tabs);
-    return html.replace(GrDiffBuilder.TAB_REGEX, htmlStr);
+  /**
+   * Takes a string of text (not HTML) and returns a string of HTML with tab
+   * elements in place of tab characters. In each case tab elements are given
+   * the width needed to reach the next tab-stop.
+   *
+   * @param {String} A line of text potentially containing tab characters.
+   * @param {Number} The width for tabs.
+   * @return {String} An HTML string potentially containing tab elements.
+   */
+  GrDiffBuilder.prototype._addTabWrappers = function(line, tabSize) {
+    if (!line.length) { return ''; }
+
+    var result = '';
+    var offset = 0;
+    var split = line.split('\t');
+    var width;
+
+    for (var i = 0; i < split.length - 1; i++) {
+      offset += split[i].length;
+      width = tabSize - (offset % tabSize);
+      result += split[i] + this._getTabWrapper(width, this._prefs.show_tabs);
+      offset += width;
+    }
+    if (split.length) {
+      result += split[split.length - 1];
+    }
+
+    return result;
   };
 
   GrDiffBuilder.prototype._addIntralineHighlights = function(content, html,
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder_test.html b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder_test.html
index cda4dc8..2c633f1 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-builder/gr-diff-builder_test.html
@@ -104,19 +104,29 @@
           '6789');
     });
 
-    test('text length with tabs', function() {
+    test('text length with tabs and unicode', function() {
       assert.equal(builder._textLength('12345', 4), 5);
       assert.equal(builder._textLength('\t\t12', 4), 10);
+      assert.equal(builder._textLength('abc💢123', 4), 7);
+
+      assert.equal(builder._textLength('abc\t', 8), 8);
+      assert.equal(builder._textLength('abc\t\t', 10), 20);
+      assert.equal(builder._textLength('', 10), 0);
+      assert.equal(builder._textLength('', 10), 0);
+      assert.equal(builder._textLength('abc\tde', 10), 12);
+      assert.equal(builder._textLength('abc\tde\t', 10), 20);
+      assert.equal(builder._textLength('\t\t\t\t\t', 20), 100);
     });
 
     test('tab wrapper insertion', function() {
       var html = 'abc\tdef';
       var wrapper = builder._getTabWrapper(
-          builder._prefs.tab_size,
+          builder._prefs.tab_size - 3,
           builder._prefs.show_tabs);
       assert.ok(wrapper);
       assert.isAbove(wrapper.length, 0);
-      assert.equal(builder._addTabWrappers(html), 'abc' + wrapper + 'def');
+      assert.equal(builder._addTabWrappers(html, builder._prefs.tab_size),
+          'abc' + wrapper + 'def');
       assert.throws(builder._getTabWrapper.bind(
           builder,
           // using \x3c instead of < in string so gjslint can parse
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-annotation.js b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-annotation.js
new file mode 100644
index 0000000..ca3e5ec
--- /dev/null
+++ b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-annotation.js
@@ -0,0 +1,206 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the 'License');
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an 'AS IS' BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+(function(window) {
+  'use strict';
+
+  // Prevent redefinition.
+  if (window.GrAnnotation) { return; }
+
+  // TODO(wyatta): refactor this to be <MARK> rather than <HL>.
+  var ANNOTATION_TAG = 'HL';
+
+  // Astral code point as per https://mathiasbynens.be/notes/javascript-unicode
+  var REGEX_ASTRAL_SYMBOL = /[\uD800-\uDBFF][\uDC00-\uDFFF]/;
+
+  var GrAnnotation = {
+
+    /**
+     * The DOM API textContent.length calculation is broken when the text
+     * contains Unicode. See https://mathiasbynens.be/notes/javascript-unicode .
+     * @param  {Text} A text node.
+     * @return {Number} The length of the text.
+     */
+    getLength: function(node) {
+      return node.textContent.replace(REGEX_ASTRAL_SYMBOL, '_').length;
+    },
+
+    /**
+     * Surrounds the element's text at specified range in an ANNOTATION_TAG
+     * element. If the element has child elements, the range is split and
+     * applied as deeply as possible.
+     */
+    annotateElement: function(parent, offset, length, cssClass) {
+      var nodes = [].slice.apply(parent.childNodes);
+      var node;
+      var nodeLength;
+      var subLength;
+
+      for (var i = 0; i < nodes.length; i++) {
+        node = nodes[i];
+        nodeLength = this.getLength(node);
+
+        // If the current node is completely before the offset.
+        if (nodeLength <= offset) {
+          offset -= nodeLength;
+          continue;
+        }
+
+        // Sublength is the annotation length for the current node.
+        subLength = Math.min(length, nodeLength - offset);
+
+        if (node instanceof Text) {
+          this._annotateText(node, offset, subLength, cssClass);
+        } else if (node instanceof HTMLElement) {
+          this.annotateElement(node, offset, subLength, cssClass);
+        }
+
+        // If there is still more to annotate, then shift the indices, otherwise
+        // work is done, so break the loop.
+        if (subLength < length) {
+          length -= subLength;
+          offset = 0;
+        } else {
+          break;
+        }
+      }
+    },
+
+    /**
+     * Wraps node in annotation tag with cssClass, replacing the node in DOM.
+     *
+     * @return {!Element} Wrapped node.
+     */
+    wrapInHighlight: function(node, cssClass) {
+      var hl;
+      if (node.tagName === ANNOTATION_TAG) {
+        hl = node;
+        hl.classList.add(cssClass);
+      } else {
+        hl = document.createElement(ANNOTATION_TAG);
+        hl.className = cssClass;
+        Polymer.dom(node.parentElement).replaceChild(hl, node);
+        hl.appendChild(node);
+      }
+      return hl;
+    },
+
+    /**
+     * Splits Text Node and wraps it in hl with cssClass.
+     * Wraps trailing part after split, tailing one if opt_firstPart is true.
+     *
+     * @param {!Node} node
+     * @param {number} offset
+     * @param {string} cssClass
+     * @param {boolean=} opt_firstPart
+     */
+    splitAndWrapInHighlight: function(node, offset, cssClass, opt_firstPart) {
+      if (this.getLength(node) === offset || offset === 0) {
+        return this.wrapInHighlight(node, cssClass);
+      } else {
+        if (opt_firstPart) {
+          this.splitNode(node, offset);
+          // Node points to first part of the Text, second one is sibling.
+        } else {
+          node = this.splitNode(node, offset);
+        }
+        return this.wrapInHighlight(node, cssClass);
+      }
+    },
+
+    /**
+     * Splits Node at offset.
+     * If Node is Element, it's cloned and the node at offset is split too.
+     *
+     * @param {!Node} node
+     * @param {number} offset
+     * @return {!Node} Trailing Node.
+     */
+    splitNode: function(element, offset) {
+      if (element instanceof Text) {
+        return this.splitTextNode(element, offset);
+      }
+      var tail = element.cloneNode(false);
+      element.parentElement.insertBefore(tail, element.nextSibling);
+      // Skip nodes before offset.
+      var node = element.firstChild;
+      while (node &&
+          this.getLength(node) <= offset ||
+          this.getLength(node) === 0) {
+        offset -= this.getLength(node);
+        node = node.nextSibling;
+      }
+      if (this.getLength(node) > offset) {
+        tail.appendChild(this.splitNode(node, offset));
+      }
+      while (node.nextSibling) {
+        tail.appendChild(node.nextSibling);
+      }
+      return tail;
+    },
+
+    /**
+     * Node.prototype.splitText Unicode-valid alternative. 
+     *
+     * DOM Api for splitText() is broken for Unicode:
+     * https://mathiasbynens.be/notes/javascript-unicode
+     *
+     * @param {!Text} node
+     * @param {number} offset
+     * @return {!Text} Trailing Text Node.
+     */
+    splitTextNode: function(node, offset) {
+      if (node.textContent.match(REGEX_ASTRAL_SYMBOL)) {
+        // TODO (viktard): Polyfill Array.from for IE10.
+        var head = Array.from(node.textContent);
+        var tail = head.splice(offset);
+        var parent = node.parentElement;
+        var headNode = document.createTextNode(head.join(''));
+        parent.replaceChild(headNode, node);
+        var tailNode = document.createTextNode(tail.join(''));
+        parent.insertBefore(tailNode, headNode.nextSibling);
+        return tailNode;
+      } else {
+        return node.splitText(offset);
+      }
+    },
+
+    _annotateText: function(node, offset, length, cssClass) {
+      var nodeLength = this.getLength(node);
+
+      // There are four cases:
+      //  1) Entire node is highlighted.
+      //  2) Highlight is at the start.
+      //  3) Highlight is at the end.
+      //  4) Highlight is in the middle.
+
+      if (offset === 0 && nodeLength === length) {
+        // Case 1.
+        this.wrapInHighlight(node, cssClass);
+      } else if (offset === 0) {
+        // Case 2.
+        this.splitAndWrapInHighlight(node, length, cssClass, true);
+      } else if (offset + length === nodeLength) {
+        // Case 3
+        this.splitAndWrapInHighlight(node, offset, cssClass, false);
+      } else {
+        // Case 4
+        this.splitAndWrapInHighlight(this.splitTextNode(node, offset), length,
+            cssClass, true);
+      }
+    },
+  };
+
+  window.GrAnnotation = GrAnnotation;
+})(window);
+
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.html b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.html
index bc3b23f..e44086e 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.html
@@ -36,5 +36,6 @@
       <content></content>
     </div>
   </template>
+  <script src="gr-annotation.js"></script>
   <script src="gr-diff-highlight.js"></script>
 </dom-module>
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.js b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.js
index c4e79eb..47a02a6 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight.js
@@ -333,107 +333,7 @@
       if (node instanceof Element && node.classList.contains('content')) {
         return this._getLength(node.querySelector('.contentText'));
       } else {
-        // DOM API for textContent.length is broken for Unicode:
-        // https://mathiasbynens.be/notes/javascript-unicode
-        return node.textContent.replace(REGEX_ASTRAL_SYMBOL, '_').length;
-      }
-    },
-
-    /**
-     * Wraps node in hl tag with cssClass, replacing the node in DOM.
-     *
-     * @return {!Element} Wrapped node.
-     */
-    _wrapInHighlight: function(node, cssClass) {
-      var hl;
-      if (node.tagName === 'HL') {
-        hl = node;
-        hl.classList.add(cssClass);
-      } else {
-        hl = document.createElement('hl');
-        hl.className = cssClass;
-        Polymer.dom(node.parentElement).replaceChild(hl, node);
-        hl.appendChild(node);
-      }
-      return hl;
-    },
-
-    /**
-     * Node.prototype.splitText Unicode-valid alternative.
-     *
-     * @param {!Text} node
-     * @param {number} offset
-     * @return {!Text} Trailing Text Node.
-     */
-    _splitTextNode: function(node, offset) {
-      if (node.textContent.match(REGEX_ASTRAL_SYMBOL)) {
-        // DOM Api for splitText() is broken for Unicode:
-        // https://mathiasbynens.be/notes/javascript-unicode
-        // TODO (viktard): Polyfill Array.from for IE10.
-        var head = Array.from(node.textContent);
-        var tail = head.splice(offset);
-        var parent = node.parentElement;
-        var headNode = document.createTextNode(head.join(''));
-        parent.replaceChild(headNode, node);
-        var tailNode = document.createTextNode(tail.join(''));
-        parent.insertBefore(tailNode, headNode.nextSibling);
-        return tailNode;
-      } else {
-        return node.splitText(offset);
-      }
-    },
-
-    /**
-     * Split Node at offset.
-     * If Node is Element, it's cloned and the node at offset is split too.
-     *
-     * @param {!Node} node
-     * @param {number} offset
-     * @return {!Node} Trailing Node.
-     */
-    _splitNode: function(element, offset) {
-      if (element instanceof Text) {
-        return this._splitTextNode(element, offset);
-      }
-      var tail = element.cloneNode(false);
-      element.parentElement.insertBefore(tail, element.nextSibling);
-      // Skip nodes before offset.
-      var node = element.firstChild;
-      while (node &&
-          this._getLength(node) <= offset ||
-          this._getLength(node) === 0) {
-        offset -= this._getLength(node);
-        node = node.nextSibling;
-      }
-      if (this._getLength(node) > offset) {
-        tail.appendChild(this._splitNode(node, offset));
-      }
-      while (node.nextSibling) {
-        tail.appendChild(node.nextSibling);
-      }
-      return tail;
-    },
-
-    /**
-     * Split Text Node and wrap it in hl with cssClass.
-     * Wraps trailing part after split, tailing one if opt_firstPart is true.
-     *
-     * @param {!Node} node
-     * @param {number} offset
-     * @param {string} cssClass
-     * @param {boolean=} opt_firstPart
-     */
-    _splitAndWrapInHighlight: function(node, offset, cssClass, opt_firstPart) {
-      if (this._getLength(node) === offset || offset === 0) {
-        return this._wrapInHighlight(node, cssClass);
-      } else {
-        if (opt_firstPart) {
-          this._splitNode(node, offset);
-          // Node points to first part of the Text, second one is sibling.
-        } else {
-          node = this._splitNode(node, offset);
-        }
-        return this._wrapInHighlight(node, cssClass);
+        return GrAnnotation.getLength(node);
       }
     },
 
@@ -470,11 +370,11 @@
 
       // Split Text node.
       if (startNode instanceof Text) {
-        startNode =
-            this._splitAndWrapInHighlight(startNode, startOffset, cssClass);
+        startNode = GrAnnotation.splitAndWrapInHighlight(
+            startNode, startOffset, cssClass);
         // Edge case: single line, text node wraps the highlight.
         if (isOneLine && this._getLength(startNode) > length) {
-          var extra = this._splitTextNode(startNode.firstChild, length);
+          var extra = GrAnnotation.splitTextNode(startNode.firstChild, length);
           startContent.insertBefore(extra, startNode.nextSibling);
           startContent.normalize();
         }
@@ -483,10 +383,10 @@
           // Edge case: single line, <hl> wraps the highlight.
           // Should leave wrapping HL's content after the highlight.
           if (isOneLine && startOffset + length < this._getLength(startNode)) {
-            this._splitNode(startNode, startOffset + length);
+            GrAnnotation.splitNode(startNode, startOffset + length);
           }
-          startNode =
-              this._splitAndWrapInHighlight(startNode, startOffset, cssClass);
+          startNode = GrAnnotation.splitAndWrapInHighlight(
+              startNode, startOffset, cssClass);
         }
       } else {
         startNode = null;
@@ -524,13 +424,13 @@
       if (!endNode) { return null; }
 
       if (endNode instanceof Text) {
-        endNode =
-            this._splitAndWrapInHighlight(endNode, endOffset, cssClass, true);
+        endNode = GrAnnotation.splitAndWrapInHighlight(
+            endNode, endOffset, cssClass, true);
       } else if (endNode.tagName == 'HL') {
         if (!endNode.classList.contains(cssClass)) {
           // Split text inside HL.
           var hl = endNode;
-          endNode = this._splitAndWrapInHighlight(
+          endNode = GrAnnotation.splitAndWrapInHighlight(
               endNode, endOffset, cssClass, true);
           if (hl.textContent.length === 0) {
             hl.remove();
@@ -627,7 +527,7 @@
           if (threadEl) {
             content.appendChild(threadEl);
           }
-          this._wrapInHighlight(text, cssClass);
+          GrAnnotation.wrapInHighlight(text, cssClass);
         }, this);
       }
     },
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight_test.html b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight_test.html
index ddc081e..88f6be5 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-highlight/gr-diff-highlight_test.html
@@ -20,6 +20,7 @@
 
 <script src="../../../bower_components/webcomponentsjs/webcomponents.min.js"></script>
 <script src="../../../bower_components/web-component-tester/browser.js"></script>
+<script src="gr-annotation.js"></script>
 
 <link rel="import" href="../../../bower_components/iron-test-helpers/iron-test-helpers.html">
 <link rel="import" href="gr-diff-highlight.html">
@@ -891,6 +892,159 @@
         assert.equal(getActionSide(), 'left');
       });
 
+      suite('annotation', function() {
+        var str;
+        var parent;
+        var textNode;
+
+        setup(function() {
+          str = 'Lorem ipsum dolor sit amet, suspendisse inceptos vehicula';
+          parent = document.createElement('div');
+          parent.appendChild(document.createTextNode(str));
+          textNode = parent.childNodes[0];
+        });
+
+        test('library is loaded', function() {
+          assert.isOk(GrAnnotation);
+        });
+
+        test('_annotateText Case 1', function() {
+          assert.equal(parent.childNodes.length, 1);
+          assert.instanceOf(parent.childNodes[0], Text);
+
+          GrAnnotation._annotateText(textNode, 0, str.length, 'foobar');
+
+          assert.equal(parent.childNodes.length, 1);
+          assert.instanceOf(parent.childNodes[0], HTMLElement);
+          assert.equal(parent.childNodes[0].className, 'foobar');
+          assert.instanceOf(parent.childNodes[0].childNodes[0], Text);
+          assert.equal(parent.childNodes[0].childNodes[0].textContent, str);
+        });
+
+        test('_annotateText Case 2', function() {
+          var length = 12;
+          var substr = str.substr(0, length);
+          var remainder = str.substr(length);
+
+          assert.equal(parent.childNodes.length, 1);
+          assert.instanceOf(parent.childNodes[0], Text);
+
+          GrAnnotation._annotateText(textNode, 0, length, 'foobar');
+
+          assert.equal(parent.childNodes.length, 2);
+
+          assert.instanceOf(parent.childNodes[0], HTMLElement);
+          assert.equal(parent.childNodes[0].className, 'foobar');
+          assert.instanceOf(parent.childNodes[0].childNodes[0], Text);
+          assert.equal(parent.childNodes[0].childNodes[0].textContent, substr);
+
+          assert.instanceOf(parent.childNodes[1], Text);
+          assert.equal(parent.childNodes[1].textContent, remainder);
+        });
+
+        test('_annotateText Case 3', function() {
+          var index = 12;
+          var length = str.length - index;
+          var remainder = str.substr(0, index);
+          var substr = str.substr(index);
+
+          assert.equal(parent.childNodes.length, 1);
+          assert.instanceOf(parent.childNodes[0], Text);
+
+          GrAnnotation._annotateText(textNode, index, length, 'foobar');
+
+          assert.equal(parent.childNodes.length, 2);
+
+          assert.instanceOf(parent.childNodes[0], Text);
+          assert.equal(parent.childNodes[0].textContent, remainder);
+
+          assert.instanceOf(parent.childNodes[1], HTMLElement);
+          assert.equal(parent.childNodes[1].className, 'foobar');
+          assert.instanceOf(parent.childNodes[1].childNodes[0], Text);
+          assert.equal(parent.childNodes[1].childNodes[0].textContent, substr);
+        });
+
+        test('_annotateText Case 4', function() {
+          var index = str.indexOf('dolor');
+          var length = 'dolor '.length;
+
+          var remainderPre = str.substr(0, index);
+          var substr = str.substr(index, length);
+          var remainderPost = str.substr(index + length);
+
+          assert.equal(parent.childNodes.length, 1);
+          assert.instanceOf(parent.childNodes[0], Text);
+
+          GrAnnotation._annotateText(textNode, index, length, 'foobar');
+
+          assert.equal(parent.childNodes.length, 3);
+
+          assert.instanceOf(parent.childNodes[0], Text);
+          assert.equal(parent.childNodes[0].textContent, remainderPre);
+
+          assert.instanceOf(parent.childNodes[1], HTMLElement);
+          assert.equal(parent.childNodes[1].className, 'foobar');
+          assert.instanceOf(parent.childNodes[1].childNodes[0], Text);
+          assert.equal(parent.childNodes[1].childNodes[0].textContent, substr);
+
+          assert.instanceOf(parent.childNodes[2], Text);
+          assert.equal(parent.childNodes[2].textContent, remainderPost);
+        });
+
+        test('_annotateElement design doc example', function() {
+          var layers = [
+            'amet, ',
+            'inceptos ',
+            'amet, ',
+            'et, suspendisse ince'
+          ];
+
+          // Apply the layers successively.
+          layers.forEach(function(layer, i) {
+            GrAnnotation.annotateElement(
+              parent, str.indexOf(layer), layer.length, 'layer-' + (i + 1));
+          });
+
+          assert.equal(parent.textContent, str);
+
+          // Layer 1:
+          var layer1 = parent.querySelectorAll('.layer-1');
+          assert.equal(layer1.length, 1);
+          assert.equal(layer1[0].textContent, layers[0]);
+          assert.equal(layer1[0].parentElement, parent);
+
+          // Layer 2:
+          var layer2 = parent.querySelectorAll('.layer-2');
+          assert.equal(layer2.length, 1);
+          assert.equal(layer2[0].textContent, layers[1]);
+          assert.equal(layer2[0].parentElement, parent);
+
+          // Layer 3:
+          var layer3 = parent.querySelectorAll('.layer-3');
+          assert.equal(layer3.length, 1);
+          assert.equal(layer3[0].textContent, layers[2]);
+          assert.equal(layer3[0].parentElement, layer1[0]);
+
+          // Layer 4:
+          var layer4 = parent.querySelectorAll('.layer-4');
+          assert.equal(layer4.length, 3);
+
+          assert.equal(layer4[0].textContent, 'et, ');
+          assert.equal(layer4[0].parentElement, layer3[0]);
+
+          assert.equal(layer4[1].textContent, 'suspendisse ');
+          assert.equal(layer4[1].parentElement, parent);
+
+          assert.equal(layer4[2].textContent, 'ince');
+          assert.equal(layer4[2].parentElement, layer2[0]);
+
+          assert.equal(layer4[0].textContent +
+              layer4[1].textContent +
+              layer4[2].textContent,
+              layers[3]);
+        });
+      });
+
       // TODO (viktard): Selection starts in line number.
       // TODO (viktard): Empty lines in selection start.
       // TODO (viktard): Empty lines in selection end.