Don't use non-0 Timers for async tests.

tests/lib/async/stream_subscription_cancel_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 the StreamSubscription.cancel return Future.
 library stream_subscription_cancel;
 
 import 'dart:async';
 import 'package:unittest/unittest.dart';
 
 void main() {
   test('subscription.cancel', () {
     var completer = new Completer();
     StreamController controller = new StreamController(
         onCancel: () => completer.future);
 
     bool done = false;
     expect(controller.stream.listen(null).cancel().then((_) => done),
            completion(equals(true)));
 
-    new Timer(const Duration(milliseconds: 10), () {
+    Timer.run(() {
       done = true;
       completer.complete();
     });
   });
 
   test('subscription.cancel after close', () {
     var completer = new Completer();
     StreamController controller = new StreamController(
         onCancel: completer.complete);
 
@@ skipping 97 matching lines @@
 
     void onDone() {
       fail("onDone is unexpected");
     }
     controller.stream
       .listen(null, onDone: onDone)
       .cancel();
     expect(doneCompleter.future, completion(equals(true)));
   });
 }