Remove package:unittest from some tests

tests/lib/async/multiple_timer_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 multiple_timer_test;
 
 import 'dart:async';
-import 'package:unittest/unittest.dart';
+import 'package:test/test.dart';
 
 const Duration TIMEOUT1 = const Duration(seconds: 1);
 const Duration TIMEOUT2 = const Duration(seconds: 2);
 const Duration TIMEOUT3 = const Duration(milliseconds: 500);
 const Duration TIMEOUT4 = const Duration(milliseconds: 1500);
 
 // The stopwatch is more precise than the Timer.
 // Some browsers (Firefox and IE so far) can trigger too early. So we add more
 // margin. We use identical(1, 1.0) as an easy way to know if the test is
 // compiled by dart2js.
@@ skipping 46 matching lines @@
     _stopwatch1.start();
     new Timer(TIMEOUT1, expectAsync(timeoutHandler1));
     _stopwatch2.start();
     new Timer(TIMEOUT2, expectAsync(timeoutHandler2));
     _stopwatch3.start();
     new Timer(TIMEOUT3, expectAsync(timeoutHandler3));
     _stopwatch4.start();
     new Timer(TIMEOUT4, expectAsync(timeoutHandler4));
   });
 }