Remove obsolete test

pkg/mutation_observer/test/mutation_observer_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.
 
 library mutation_observer_test;
 
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
@@ skipping 83 matching lines @@
       var container = new DivElement();
       var div1 = new DivElement();
       var div2 = new DivElement();
       var mutationObserver = new MutationObserver(
           mutationCallback(2, orderedEquals([div1, div2])));
       mutationObserver.observe(container, childList: true, subtree: true);
 
       container.append(div1);
       div1.append(div2);
     });
-
-    test('mutation event', () {
-      var event = new MutationEvent('something', prevValue: 'prev',
-          newValue: 'new', attrName: 'attr');
-      expect(event is MutationEvent, isTrue);
-      expect(event.prevValue, 'prev');
-      expect(event.newValue, 'new');
-      expect(event.attrName, 'attr');
-    });
   });
 }