Convert polygerrit to es6-modules

This change replace all HTML imports with es6-modules. The only exceptions are:
* gr-app.html file, which can be deleted only after updating the
  gerrit/httpd/raw/PolyGerritIndexHtml.soy file.
* dark-theme.html which is loaded via importHref. Must be updated manually
  later in a separate change.

This change was produced automatically by ./es6-modules-converter.sh script.
No manual changes were made.

Change-Id: I0c447dd8c05757741e2c940720652d01d9fb7d67
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.js b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.js
index 47cf771..5fe516c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-mode-selector/gr-diff-mode-selector_html.js
@@ -1,28 +1,22 @@
-<!--
-@license
-Copyright (C) 2018 The Android Open Source Project
+/**
+ * @license
+ * Copyright (C) 2020 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.
+ */
+import {html} from '@polymer/polymer/lib/utils/html-tag.js';
 
-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.
--->
-
-<link rel="import" href="/bower_components/polymer/polymer.html">
-<link rel="import" href="/bower_components/iron-icon/iron-icon.html">
-<link rel="import" href="../../../styles/shared-styles.html">
-<link rel="import" href="../../shared/gr-button/gr-button.html">
-<link rel="import" href="../../shared/gr-rest-api-interface/gr-rest-api-interface.html">
-
-<dom-module id="gr-diff-mode-selector">
-  <template>
+export const htmlTemplate = html`
     <style include="shared-styles">
       :host {
         /* Used to remove horizontal whitespace between the icons. */
@@ -36,25 +30,11 @@
         width: 1.3rem;
       }
     </style>
-    <gr-button
-        id="sideBySideBtn"
-        link
-        has-tooltip
-        class$="[[_computeSelectedClass(mode, _VIEW_MODES.SIDE_BY_SIDE)]]"
-        title="Side-by-side diff"
-        on-click="_handleSideBySideTap">
+    <gr-button id="sideBySideBtn" link="" has-tooltip="" class\$="[[_computeSelectedClass(mode, _VIEW_MODES.SIDE_BY_SIDE)]]" title="Side-by-side diff" on-click="_handleSideBySideTap">
       <iron-icon icon="gr-icons:side-by-side"></iron-icon>
     </gr-button>
-    <gr-button
-        id="unifiedBtn"
-        link
-        has-tooltip
-        title="Unified diff"
-        class$="[[_computeSelectedClass(mode, _VIEW_MODES.UNIFIED)]]"
-        on-click="_handleUnifiedTap">
+    <gr-button id="unifiedBtn" link="" has-tooltip="" title="Unified diff" class\$="[[_computeSelectedClass(mode, _VIEW_MODES.UNIFIED)]]" on-click="_handleUnifiedTap">
       <iron-icon icon="gr-icons:unified"></iron-icon>
     </gr-button>
     <gr-rest-api-interface id="restAPI"></gr-rest-api-interface>
-  </template>
-  <script src="gr-diff-mode-selector.js"></script>
-</dom-module>
+`;