Add safetymargin.

tests/lib/async/stream_periodic3_test.dart

 // Copyright (c) 2013, 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.
 
 // Test merging streams.
 library dart.test.stream_from_iterable;
 
 import "dart:async";
 import '../../../pkg/unittest/lib/unittest.dart';
 
+// The stopwatch is more precise than the Timer. It can happen that
+// the TIMEOUT triggers *slightly* too early on the VM. So we add a millisecond
+// as safetymargin.
+// 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.
+int get safetyMargin => identical(1, 1.0) ? 5 : 1;
+
 main() {
   test("stream-periodic3", () {
     Stopwatch watch = new Stopwatch()..start();
     Stream stream = new Stream.periodic(const Duration(milliseconds: 1),
                                         (x) => x);
     stream.take(10).listen((_) { }, onDone: expectAsync0(() {
-      int microsecs = watch.elapsedMicroseconds;
-      // Give it some slack. The Stopwatch is more precise than the timers.
-      int millis = (microsecs + 200) ~/ 1000;
+      int millis = watch.elapsedMilliseconds + safetyMargin;
       expect(millis, greaterThan(10));
     }));
   });
 }