Fix for postMessage on dart2js

lib/dom/src/frog_DOMImplementation.dart

Index: lib/dom/src/frog_DOMImplementation.dart
diff --git a/lib/dom/src/frog_DOMImplementation.dart b/lib/dom/src/frog_DOMImplementation.dart
index f0b006686841f09cd305f6a293914871e81f3eae..86bac7b22839fcf1644073b1cce0762f2db5f535 100644
--- a/lib/dom/src/frog_DOMImplementation.dart
+++ b/lib/dom/src/frog_DOMImplementation.dart
@@ -31,12 +31,23 @@ class _DOMWindowCrossFrameImpl implements DOMType, DOMWindow {
                    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);