Fix MessageEvent.data to convert using structured clone.

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 cf3c4e4447f7de4a98c9b85915a431512a5182eb..1b3280d287707f799102e3dfadde9311934e40b5 100644
--- a/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
+++ b/tools/dom/templates/html/impl/impl_MessageEvent.darttemplate
@@ -21,11 +21,22 @@ $if DART2JS
           type, canBubble, cancelable, data, origin, lastEventId, source,
           messagePorts);
     }
-$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
 }