Remove package:unittest from some tests

tests/lib/async/timer_isActive_test.dart

 // Copyright (c) 2011, 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.
 
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 main() {
 
   test("timer isActive test", () {
     Timer t;
 
     t = new Timer(const Duration(seconds: 1),
                   expectAsync(() => expect(t.isActive, equals(false))));
     expect(t.isActive, equals(true));
   });
@@ skipping 19 matching lines @@
   test("timer cancel test", () {
     Timer timer = new Timer(const Duration(seconds: 15),
                             () => fail("Should not be reached."));
     Timer.run(expectAsync(() {
       expect(timer.isActive, equals(true));
       timer.cancel();
       expect(timer.isActive, equals(false));
     }));
   });
 }