Run new MessageEvent constructor instead of initMessageEvent in Firefox.

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

Index: tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
diff --git a/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
index 8a16f106de30936f6fb745a34260478fcdbdd94e..621d28a0a35264ff9ad097f51bc98eb748ae0fab 100644
--- a/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
@@ -14,6 +14,13 @@ $(ANNOTATIONS)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS$NATIVESPEC {
     if (source == null) {
       source = window;
     }
+$if DART2JS
+    if (!Device.isIE) {
+      return JS('MessageEvent', 'new MessageEvent(#, {bubbles: #, cancelable: #, data: #, origin: #, lastEventId: #, source: #, ports: #})',
+          type, canBubble, cancelable, data, origin, lastEventId, source,
+          messagePorts);
+    }
+$endif 
     var event = document._createEvent("MessageEvent");
     event._initMessageEvent(type, canBubble, cancelable, data, origin,
         lastEventId, source, messagePorts);