Mark addEventListeners as Unsupported when called for cross-frame window actions.

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index 15967d9542aafdadef752de74c18407fda2f3671..82454e38792a675bb78e6030adc5a871deec3470 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -35411,15 +35411,19 @@ class _DOMWindowCrossFrame implements WindowBase {
   }
 
   // TODO(efortuna): Remove this method. dartbug.com/16814
-  Events get on => throw new UnimplementedError();
+  Events get on => throw new UnsupportedError(
+    'You can only attach EventListeners to your own window.');
   // TODO(efortuna): Remove this method. dartbug.com/16814
   void addEventListener(String type, EventListener listener, [bool useCapture])
-      => throw new UnimplementedError();
+      => throw new UnsupportedError(
+        'You can only attach EventListeners to your own window.');
   // TODO(efortuna): Remove this method. dartbug.com/16814
-  bool dispatchEvent(Event event) => throw new UnimplementedError();
+  bool dispatchEvent(Event event) => throw new UnsupportedError(
+    'You can only attach EventListeners to your own window.');
   // TODO(efortuna): Remove this method. dartbug.com/16814
   void removeEventListener(String type, EventListener listener,
-      [bool useCapture]) => throw new UnimplementedError();
+      [bool useCapture]) => throw new UnsupportedError(
+          'You can only attach EventListeners to your own window.');
 }
 
 class _LocationCrossFrame implements LocationBase {