Use Stopwatch instead of DateTime to measure elapsed time in tests.

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 'package:unittest/unittest.dart';
 
 const Duration TIMEOUT = const Duration(milliseconds: 100);
 
 // Some browsers (Firefox and IE so far) can trigger too early. Add a safety
 // margin. We use identical(1, 1.0) as an easy way to know if the test is
 // compiled by dart2js.
 int get safetyMargin => identical(1, 1.0) ? 100 : 0;
 
 createTimer(replyTo) {
   new Timer(TIMEOUT, () {
     replyTo.send("timer_fired");
   });
 }
 
 main() {
   test("timer in isolate", () {
-    int startTime;
-    int endTime;
-
+    Stopwatch stopwatch = new Stopwatch();
     ReceivePort port = new ReceivePort();
 
     port.first.then(expectAsync((msg) {
       expect("timer_fired", msg);
-      int endTime = (new DateTime.now()).millisecondsSinceEpoch;
-      expect(endTime - startTime + safetyMargin, greaterThanOrEqualTo(TIMEOUT.inMilliseconds));
+      expect(stopwatch.elapsedMilliseconds + safetyMargin,
+             greaterThanOrEqualTo(TIMEOUT.inMilliseconds));
     }));
 
-    startTime = (new DateTime.now()).millisecondsSinceEpoch;
+    stopwatch.start();
     var remote = Isolate.spawn(createTimer, port.sendPort);
   });
 }