Remove package:unittest from some tests

tests/lib/async/timer_cancel2_test.dart

 // Copyright (c) 2012, 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 timer_cancel2_test;
 
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 main() {
   // Test that a timeout handler can cancel itself.
   test("timer cancel test 2", () {
     var cancelTimer;
 
     void cancelHandler(Timer timer) {
       cancelTimer.cancel();
     }
 
     cancelTimer = new Timer.periodic(const Duration(milliseconds: 1),
                                      expectAsync(cancelHandler));
   });
 }