Copy lib/dom tests that use dart:html to lib/html.

tests/dom/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);
-    });
-}