Run new MessageEvent constructor instead of initMessageEvent in Firefox.

tools/dom/templates/html/impl/impl_MessageEvent.darttemplate

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // WARNING: Do not edit - generated code.
 
 part of $LIBRARYNAME;
 
 $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
   factory $CLASSNAME(String type,
       {bool canBubble: false, bool cancelable: false, Object data,
       String origin, String lastEventId,
       Window source, List messagePorts}) {
     if (source == null) {
       source = window;
     }
+$if DART2JS
+    if (Device.isFirefox) {

[blois, 2014/06/30 20:54:31]

If their change was done to follow the spec, is this going to break elsewhere as
well?

new MessageEvent appears to work on Chrome and Safari, untested on IE.

+      return JS('MessageEvent', 'new MessageEvent(#, {bubbles: #, cancelable: #, data: #, origin: #, lastEventId: #, source: #, ports: #})',
+          type, canBubble, cancelable, data, origin, lastEventId, source,
+          messagePorts);
+    }
+$endif 
     var event = document._createEvent("MessageEvent");
     event._initMessageEvent(type, canBubble, cancelable, data, origin,
         lastEventId, source, messagePorts);
     return event;
   }
 $!MEMBERS
 }