Fixing iframe.contentWindow.postMessage passing incorrect data

tests/html/postmessage_structured_test.dart

Index: tests/html/postmessage_structured_test.dart
diff --git a/tests/html/postmessage_structured_test.dart b/tests/html/postmessage_structured_test.dart
index 9ceeebca3a174b57f69dfff587e67493709b2742..1dc65d7549d6e29dac2caef33d5f93866e9a564a 100644
--- a/tests/html/postmessage_structured_test.dart
+++ b/tests/html/postmessage_structured_test.dart
@@ -149,4 +149,17 @@ main() {
     go('typed_arrays_list', typed_arrays_list);
   });
 
+  group('iframe', () {
+    test('postMessage clones data', () {
+      var iframe = new IFrameElement();
+      var future = iframe.onLoad.first.then((_) {
+        iframe.contentWindow.postMessage(new HashMap<String,num>(), '*');
+      });
+      iframe.src = 'about:blank';
+      document.body.append(iframe);
+
+      return future;
+    });
+  });
+
 }