Revert "Dartium JS Interop enabled."

tests/html/messageevent_test.dart

 library SerializedScriptValueTest;
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
   test('new MessageEvent', () {
       final event = new MessageEvent('type', cancelable: true, data: 'data',
           origin: 'origin', lastEventId: 'lastEventId');
 
       expect(event.type, equals('type'));
       expect(event.bubbles, isFalse);
       expect(event.cancelable, isTrue);
       expect(event.data, equals('data'));
       expect(event.origin, equals('origin'));
       // IE allows setting this but just ignores it.
       // expect(event.lastEventId, equals('lastEventId'));
-      expect(event.source, window);
+      expect(event.source, same(window));
       // TODO(antonm): accessing ports is not supported yet.
   });
 }