Fix timer test

tests/isolate/timer_isolate_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:isolate';
+import 'dart:async';
 import '../../pkg/unittest/lib/unittest.dart';
 
 const int TIMEOUT = 100;
 
 createTimer() {
   port.receive((msg, replyTo) {
     new Timer(TIMEOUT, (_) {
       replyTo.send("timer_fired");
     });
   });
 }
 
 main() {
   test("timer in isolate", () {
     int startTime;
     int endTime;
     
     port.receive(expectAsync2((msg, _) {
       expect("timer_fired", msg);
       int endTime = (new Date.now()).millisecondsSinceEpoch;
       expect(endTime - startTime, greaterThanOrEqualTo(TIMEOUT));
       port.close();
     }));
     
     startTime = (new Date.now()).millisecondsSinceEpoch;
     var sendPort = spawnFunction(createTimer);
     sendPort.send("sendPort", port.toSendPort());
   });
 }