Minimal fix to get cross frame tests working, take #2.

lib/html/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 Window {
+  // Private window.
+  _WindowImpl _window;
+
+  // Fields.
+  // TODO(vsm): Implement history and location getters.
+
+  bool get closed() => _window.closed;
+  int get length() => _window.length;
+  Window get opener() => _createSafe(_window.opener);
+  Window get parent() => _createSafe(_window.parent);
+  Window get top() => _createSafe(_window.top);
+
+  // Methods.
+  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);
+    } else {
+      _window.postMessage(message, targetOrigin, messagePorts);
+    }
+  }
+
+  // Implementation support.
+  _DOMWindowCrossFrameImpl(this._window);
+
+  static Window _createSafe(w) {
+    if (w === window) {
+      return w;
+    } else {
+      // TODO(vsm): Cache or implement equality.
+      return new _DOMWindowCrossFrameImpl(w);
+    }
+  }
+}