Remove package:unittest from some tests

tests/lib/async/timer_cancel_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_cancel_test;
 
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 main() {
   final ms = const Duration(milliseconds: 1);
   test("simple timer", () {
     Timer cancelTimer;
     int repeatTimer;
 
     void unreachable() {
       fail("should not be reached");
     }
@@ skipping 15 matching lines @@
     repeatTimer = 0;
     new Timer.periodic(ms * 1500, expectAsync(repeatHandler));
   });
 
   test("cancel timer with same time", () {
     var t2;
     var t1 = new Timer(ms * 0, expectAsync(() => t2.cancel()));
     t2 = new Timer(ms * 0, expectAsync(t1.cancel, count: 0));
   });
 }