fix barback and polymer build warnings

pkg/scheduled_test/lib/src/mock_clock.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.
 
 /// A library that wraps [Timer] in a way that can be mocked out in test code.
 /// Application code only needs to use [newTimer] to get an instance of [Timer].
 /// Then test code can call [mock] to mock out all new [Timer] instances so that
 /// they're controllable by a returned [Clock] object.
 library mock_clock;
 
@@ skipping 14 matching lines @@
   if (_mocked) {
     throw new StateError("mock_clock.mock() has already been called.");
   }
 
   _clock = new Clock._();
   return _clock;
 }
 
 typedef void TimerCallback();
 
-/// Returns a new (possibly mocked) [Timer]. Works the same as [new Timer].
+/// Returns a new (possibly mocked) [Timer]. Works the same as `new Timer`.

[nweiz, 2013/10/30 19:28:39]

Also reference the issue number here.

 Timer newTimer(Duration duration, TimerCallback callback) =>
   _mocked ? new _MockTimer(duration, callback) : new Timer(duration, callback);
 
 /// A clock that controls when mocked [Timer]s move forward in time. It starts
 /// at time 0 and advances forward millisecond-by-millisecond, broadcasting each
 /// tick on the [onTick] stream.
 class Clock {
   /// The current time of the clock, in milliseconds. Starts at 0.
   int get time => _time;
   int _time = 0;
@@ skipping 19 matching lines @@
       });
     }
   }
 
   /// Automatically progresses forward in time as long as there are still
   /// subscribers to [onTick] (that is, [Timer]s waiting to fire). After each
   /// tick, this pumps the event loop repeatedly so that all non-clock-dependent
   /// code runs before the next tick.
   void run() {
     pumpEventQueue().then((_) {
-      if (!_broadcastController.hasListener) return;
+      if (!_broadcastController.hasListener) return null;
       tick();
       return run();
     });
   }
 }
 
 /// A mock implementation of [Timer] that uses [Clock] to keep time, rather than
 /// the system clock.
 class _MockTimer implements Timer {
   /// The time at which the timer should fire.
   final int _time;
 
   /// The callback to run when the timer fires.
   final TimerCallback _callback;
 
   /// The subscription to the [Clock.onTick] stream.
   StreamSubscription _subscription;
 
   _MockTimer(Duration duration, this._callback)
       : _time = _clock.time + duration.inMilliseconds {
     _subscription = _clock.onTick.listen((time) {
       if (time < _time) return;
       _subscription.cancel();
       _callback();
     });
   }
 
-  void cancel() => _subscription.cancel();
+  bool get isActive => _subscription != null;
+
+  void cancel() {
+    _subscription.cancel();
+    _subscription = null;
+  }
 }