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

tests/html/cross_frame_test.dart

Index: tests/html/cross_frame_test.dart
diff --git a/tests/html/cross_frame_test.dart b/tests/html/cross_frame_test.dart
index 8269e284887f6ce5d9ce303f9c8f62ee64f6ba7c..53b9ddb6b50154c1c41425669680bcb14a4551b3 100644
--- a/tests/html/cross_frame_test.dart
+++ b/tests/html/cross_frame_test.dart
@@ -22,4 +22,12 @@ main() {
       // 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);
+    });
 }