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;
+  var _window;

[vsm, 2012/09/28 21:02:42]

This is a window to another frame.  It's proto is not patched accordingly, so
dart2js doesn't recognize this as a _WindowImpl.  It was previously causing
checked mode failures.

[sra1, 2012/10/02 21:14:47]

Say this in a comment.
"Declared var since Window from another frame has no Dart type."

 
   // 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);
     }
   }
 }