Run new MessageEvent constructor instead of initMessageEvent in Firefox.

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 3a1e464a3ce902b7ed2fa402ec09a573789d4a18..953eb6ccd03666c92c789b743861d70626f20c6c 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -18415,6 +18415,11 @@ class MessageEvent extends Event native "MessageEvent" {
     if (source == null) {
       source = window;
     }
+    if (Device.isFirefox) {
+      return JS('MessageEvent', 'new MessageEvent(#, {bubbles: #, cancelable: #, data: #, origin: #, lastEventId: #, source: #, ports: #})',
+          type, canBubble, cancelable, data, origin, lastEventId, source,
+          messagePorts);
+    }
     var event = document._createEvent("MessageEvent");
     event._initMessageEvent(type, canBubble, cancelable, data, origin,
         lastEventId, source, messagePorts);