assert: Include description of duplicate key registration

This way its easier to track down the problem during GWT debug
sessions.

Change-Id: Idab1f1163069cba7285f986dcfebd34391b9045c

src/main/java/com/google/gwtexpui/globalkey/client/KeyCommandSet.java

diff --git a/src/main/java/com/google/gwtexpui/globalkey/client/KeyCommandSet.java b/src/main/java/com/google/gwtexpui/globalkey/client/KeyCommandSet.java
index 05d0104..4f3205a 100644
--- a/src/main/java/com/google/gwtexpui/globalkey/client/KeyCommandSet.java
+++ b/src/main/java/com/google/gwtexpui/globalkey/client/KeyCommandSet.java
@@ -53,7 +53,9 @@
   }
 
   public void add(final KeyCommand k) {
-    assert !map.containsKey(k.keyMask);
+    assert !map.containsKey(k.keyMask)
+         : "Key " + k.describeKeyStroke().asString()
+         + " already registered";
     if (!map.containsKey(k.keyMask)) {
       map.put(k.keyMask, k);
     }