Fix MessageEvent.data to convert using structured clone.

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;
 
 $(ANNOTATIONS)$(NATIVESPEC)$(CLASS_MODIFIERS)class $CLASSNAME$EXTENDS$IMPLEMENTS {
   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;
     }
 $if DART2JS
     if (!Device.isIE) { // TODO: This if check should be removed once IE
       // implements the constructor.
       return JS('MessageEvent', 'new MessageEvent(#, {bubbles: #, cancelable: #, data: #, origin: #, lastEventId: #, source: #, ports: #})',
           type, canBubble, cancelable, data, origin, lastEventId, source,
           messagePorts);
     }
-$endif 
+$endif
     var event = document._createEvent("MessageEvent");
     event._initMessageEvent(type, canBubble, cancelable, data, origin,
         lastEventId, source, messagePorts);
     return event;
   }
+
+$if DARTIUM
+  // TODO(alanknight): This really should be generated by the
+  // _OutputConversion in the systemnative.py script, but that doesn't
+  // use those conversions right now, so do this as a one-off.
+  @DomName('MessageEvent.data')
+  @DocsEditable()
+  dynamic get data => convertNativeToDart_SerializedScriptValue(
+      _blink.BlinkMessageEvent.instance.data_Getter_(unwrap_jso(this)));
+$endif
+
 $!MEMBERS
 }