Cleaning up event constructors

tests/html/history_test.dart

 library HistoryTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_individual_config.dart';
 import 'dart:html';
 
 /// Waits for a callback once, then removes the event handler.
 void expectAsync1Once(EventListenerList list, void callback(arg)) {
   var fn = null;
   fn = expectAsync1((arg) {
     list.remove(fn);
@@ skipping 48 matching lines @@
 
     test('replaceState', () {
       expect(() {
         var length = window.history.length;
 
         window.history.replaceState(null, document.title, '?foo=baz');
         expect(window.history.length, length);
         expect(window.location.href.endsWith('foo=baz'), isTrue);
       }, expectation);
     });
+
+    test('hashchangeevent', () {
+      var event = new HashChangeEvent('change', oldUrl:'old', newUrl: 'new');
+      expect(event is HashChangeEvent, true);
+      expect(event.oldUrl, 'old');
+      expect(event.newUrl, 'new');
+    });
   });
 }