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

tests/dom/messageevent_test.dart

-#library('SerializedScriptValueTest');
-#import('../../lib/unittest/unittest.dart');
-#import('../../lib/unittest/html_config.dart');
-#import('dart:html');
-
-main() {
-  useHtmlConfiguration();
-
-  test('MessageEvent.initMessageEvent', () {
-      final event = document.$dom_createEvent('MessageEvent');
-      event.initMessageEvent('type', false, true, 'data', 'origin', 'lastEventId', window, []);
-      expect(event.type).equals('type');
-      expect(event.bubbles).equals(false);
-      expect(event.cancelable).equals(true);
-      expect(event.data).equals('data');
-      expect(event.origin).equals('origin');
-      expect(event.lastEventId).equals('lastEventId');
-      expect(event.source).same(window);
-      // TODO(antonm): accessing ports is not supported yet.
-  });
-}