Fixing iframe.contentWindow.postMessage passing incorrect data

tools/dom/src/dart2js_DOMImplementation.dart

Index: tools/dom/src/dart2js_DOMImplementation.dart
diff --git a/tools/dom/src/dart2js_DOMImplementation.dart b/tools/dom/src/dart2js_DOMImplementation.dart
index a844f232b370c8629d499be5b8bdb9734b282f63..f263def4a86e374081c680e427af725774f1afa4 100644
--- a/tools/dom/src/dart2js_DOMImplementation.dart
+++ b/tools/dom/src/dart2js_DOMImplementation.dart
@@ -32,9 +32,12 @@ class _DOMWindowCrossFrame implements WindowBase {
 
   void postMessage(var message, String targetOrigin, [List messagePorts = null]) {
     if (messagePorts == null) {
-      JS('void', '#.postMessage(#,#)', _window, message, targetOrigin);
+      JS('void', '#.postMessage(#,#)', _window,
+          convertDartToNative_SerializedScriptValue(message), targetOrigin);
     } else {
-      JS('void', '#.postMessage(#,#,#)', _window, message, targetOrigin, messagePorts);
+      JS('void', '#.postMessage(#,#,#)', _window,
+          convertDartToNative_SerializedScriptValue(message), targetOrigin,
+          messagePorts);
     }
   }