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

tools/dom/src/dart2js_DOMImplementation.dart

Index: tools/dom/src/dart2js_DOMImplementation.dart
diff --git a/tools/dom/src/dart2js_DOMImplementation.dart b/tools/dom/src/dart2js_DOMImplementation.dart
index 78a54b3fca6b048f0a38fc16e68555fc342fa307..a844f232b370c8629d499be5b8bdb9734b282f63 100644
--- a/tools/dom/src/dart2js_DOMImplementation.dart
+++ b/tools/dom/src/dart2js_DOMImplementation.dart
@@ -51,15 +51,19 @@ 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 {