Remove package:unittest from some tests

tests/lib/async/timer_cancel1_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_cancel1_test;
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 main() {
   // Test that a timeout handler can cancel another.
   test("timer cancel1 test", () {
     var canceleeTimer;
     var cancelerTimer;
 
     void unreachable() {
       fail("A canceled timeout handler should be unreachable.");
     }
 
     void handler() {
       canceleeTimer.cancel();
     }
 
     cancelerTimer = new Timer(const Duration(milliseconds: 1),
                               expectAsync(handler));
     canceleeTimer = new Timer(const Duration(milliseconds: 1000),
                               expectAsync(unreachable, count: 0));
   });
 }