Fix for postMessage on dart2js

lib/dom/src/frog_DOMImplementation.dart

 // Copyright (c) 2012, 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.
 
 // TODO(vsm): Unify with Dartium version.
 class _DOMWindowCrossFrameImpl implements DOMType, DOMWindow {
   // Private window.
   _DOMWindowJs _window;
 
   // DOMType
@@ skipping 13 matching lines @@
   void focus() => _window.focus();
 
   void blur() => _window.blur();
 
   void close() => _window.close();
 
   void postMessage(Dynamic message,
                    String targetOrigin,
                    [List messagePorts = null]) {
     if (messagePorts == null) {
-      _window.postMessage(message, targetOrigin);
+      _postMessage2(_window, message, targetOrigin);
     } else {
-      _window.postMessage(message, targetOrigin, messagePorts);
+      _postMessage3(_window, message, targetOrigin, messagePorts);
     }
   }
 
+  // TODO(vsm): This is a hack to workaround dartbug.com/3175.  We
+  // need a more robust convention to invoke JS methods on the
+  // underlying window.
+  static void _postMessage2(win, message, targetOrigin) native """
+    win.postMessage(message, targetOrigin);
+""";
+
+  static void _postMessage3(win, message, targetOrigin, messagePorts) native """
+    win.postMessage(message, targetOrigin, messagePorts);
+""";
+
   // Implementation support.
   _DOMWindowCrossFrameImpl(this._window);
 
   static DOMWindow _createSafe(w) {
     if (w === window) {
       return w;
     } else {
       // TODO(vsm): Cache or implement equality.
       return new _DOMWindowCrossFrameImpl(w);
     }
   }
 }