Remove obsolete test

pkg/mutation_observer/test/mutation_observer_test.dart

Index: pkg/mutation_observer/test/mutation_observer_test.dart
diff --git a/pkg/mutation_observer/test/mutation_observer_test.dart b/pkg/mutation_observer/test/mutation_observer_test.dart
index 24dbccf2b6e7f31a5a9dea335da8e84cf6999078..e3bbdf4bbce8ab9add54252a7be66d342ee5741b 100644
--- a/pkg/mutation_observer/test/mutation_observer_test.dart
+++ b/pkg/mutation_observer/test/mutation_observer_test.dart
@@ -101,14 +101,5 @@ testMutationObserver() {
       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');
-    });
   });
 }