Bugfix in html_dart2js: Detect window objects in a way compatible with cross domain iframes

dart/tests/html/cross_domain_iframe_test.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.
+
+import 'dart:html';
+
+import 'package:unittest/unittest.dart';
+
+main() {
+  test('cross_domain_iframe', () {
+    var uri = Uri.parse(window.location.href);
+
+    var crossOriginPort = int.parse(uri.queryParameters['crossOriginPort']);
+    var crossOrigin = '${uri.scheme}://${uri.host}:$crossOriginPort';
+    var crossOriginUrl =
+        '$crossOrigin/root_dart/tests/html/cross_domain_iframe_script.html';
+
+    var iframe = new IFrameElement();
+    iframe.src = crossOriginUrl;
+    document.body.append(iframe);
+
+    window.onMessage.where((MessageEvent event) {
+      return event.origin == crossOrigin;
+    }).first.then(expectAsync((MessageEvent event) {
+      expect(event.data, equals('foobar'));
+      expect(event.source, isNotNull);
+    }));
+  });
+}