Generate and use cross frame wrappers for types in other frames/windows.

tests/html/cross_frame_test.dart

 #library('CrossFrameTest');
 #import('../../lib/unittest/unittest.dart');
 #import('../../lib/unittest/html_config.dart');
 #import('dart:html');
 
 main() {
   useHtmlConfiguration();
 
   test('contentWindow', () {
       final iframe = new Element.tag('iframe');
       document.body.nodes.add(iframe);
       final frameWindow = iframe.contentWindow;
 
       // Ensure that the frame's document is inaccessible via window.
       Expect.throws(() => frameWindow.document);
     });
 
   test('contentDocument', () {
       final iframe = new Element.tag('iframe');
       document.body.nodes.add(iframe);
 
       // Ensure that the frame's document is inaccessible.
       Expect.throws(() => iframe.contentDocument);
     });
+
+  test('window.open', () {
+      Window win = window.open('http://dartlang.org', 'Dart');
+      Expect.isTrue(win is Window);
+
+      // Ensure that the new window's document is inaccessible.
+      Expect.throws(() => win.contentDocument);
+    });
 }