Fix the type on a wrapped window

lib/html/src/dart2js_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;
+  // Private window.  Note, this is a window in another frame, so it
+  // cannot be typed as "Window" as its prototype is not patched
+  // properly.  Its fields and methods can only be accessed via JavaScript.
+  var _window;
 
   // Fields.
   // TODO(vsm): Implement history and location getters.
 
   // TODO(vsm): Add frames to navigate subframes.  See 2312.
 
   bool get closed => _closed(_window);
   static bool _closed(win) native "return win.closed;";
 
   Window get opener => _createSafe(_opener(_window));
@@ skipping 41 matching lines @@
 
   static Window _createSafe(w) {
     if (w === window) {
       return w;
     } else {
       // TODO(vsm): Cache or implement equality.
       return new _DOMWindowCrossFrameImpl(w);
     }
   }
 }