Cleaning up event constructors

tests/html/messageevent_test.dart

 library SerializedScriptValueTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/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, []);
+      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'));
       expect(event.lastEventId, equals('lastEventId'));
       expect(event.source, same(window));
       // TODO(antonm): accessing ports is not supported yet.
   });
 }