Fixing conversion of null event targets.

tests/html/mouse_event_test.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// Regression test for dart2js, that used to emit wrong code on is
-// checks of native classes that are not instantiated.
-
-import 'dart:html';
+library mouse_event_test;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
-
-var a = [new Object()];
+import 'dart:html';
 
 main() {
   useHtmlConfiguration();
 
-  test('is', () {
-    expect(a[0] is Node, isFalse);
+  test('relatedTarget', () {
+    var event = new MouseEvent('mouseout');
+    expect(event.relatedTarget, isNull);
   });
 }