fix barback and polymer build warnings

pkg/scheduled_test/lib/src/mock_clock.dart

Index: pkg/scheduled_test/lib/src/mock_clock.dart
diff --git a/pkg/scheduled_test/lib/src/mock_clock.dart b/pkg/scheduled_test/lib/src/mock_clock.dart
index d260b78f4df5516a9da8224485eecfd0608ab893..718f47b0ba20bd7697f8e99f1100a080fd923f99 100644
--- a/pkg/scheduled_test/lib/src/mock_clock.dart
+++ b/pkg/scheduled_test/lib/src/mock_clock.dart
@@ -32,7 +32,7 @@ Clock mock() {
 
 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);
 
@@ -72,7 +72,7 @@ class Clock {
   /// code runs before the next tick.
   void run() {
     pumpEventQueue().then((_) {
-      if (!_broadcastController.hasListener) return;
+      if (!_broadcastController.hasListener) return null;
       tick();
       return run();
     });
@@ -100,5 +100,10 @@ class _MockTimer implements Timer {
     });
   }
 
-  void cancel() => _subscription.cancel();
+  bool get isActive => _subscription != null;
+
+  void cancel() {
+    _subscription.cancel();
+    _subscription = null;
+  }
 }