pkg/mutation_observer: fix use of removed unittest feature

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 37 matching lines @@
             nodes.add(node);
           }
         }
         if (nodes.length >= count) {
           done = true;
           expect(nodes.length, count);
           expect(nodes, expectation);
         }
       }
 
-      return expectAsyncUntil2(callback, () => done);
+      return expectAsyncUntil(callback, () => done);
     }
 
     test('empty options is syntax error', () {
       var mutationObserver = new MutationObserver(
           (mutations, observer) { expect(false, isTrue,
               reason: 'Should not be reached'); });
       expect(() { mutationObserver.observe(document); },
              throws);
     });
 
@@ skipping 27 matching lines @@
       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);
     });
   });
 }