Cleaning up event constructors

tests/html/events_test.dart

 library EventsTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:html';
 
 main() {
   useHtmlConfiguration();
   test('TimeStamp', () {
     Event event = new Event('test');
 
@@ skipping 36 matching lines @@
     element.on['test'].dispatch(event);
     expect(invocationCounter, 1);
 
     element.on['test'].add(handler, false);
     invocationCounter = 0;
     element.on['test'].dispatch(event);
     expect(invocationCounter, 1);
   });
   test('InitMouseEvent', () {
     DivElement div = new Element.tag('div');
-    MouseEvent event = document.$dom_createEvent('MouseEvent');
-    event.$dom_initMouseEvent('zebra', true, true, window, 0, 1, 2, 3, 4, false, false, false, false, 0, div);
+    MouseEvent event = new MouseEvent('zebra', relatedTarget: div);
   });
 }