feat(mock): Add timer queue checks in mock zone

lib/mock/zone.dart

@@ -68,6 +68,25 @@ microLeap() {
  */
 isAsyncQueueEmpty() => _asyncQueue.isEmpty;
 
+/**
+ * Returns whether there are outstanding timers.
+ */
+isTimerQueueEmpty() => _timerQueue.isEmpty;
+
+/**
+ * Returns whether there are outstanding non-periodic timers.
+ */
+isNonPeriodicTimerQueueEmpty() => _timerQueue
+    .where((_TimerSpec spec) => !spec.periodic)
+    .isEmpty;
+
+/**
+ * Returns whether there are outstanding periodic timers.
+ */
+isPeriodicTimerQueueEmpty() => _timerQueue
+    .where((_TimerSpec spec) => spec.periodic)
+    .isEmpty;
+
 /**
  * Simulates a clock tick by running any scheduled timers. Can only be used
  * in [async] tests.Clock tick will call [microLeap] to process the microtask

test/mock/zone_spec.dart

@@ -332,6 +332,28 @@ void main() {
                 (_) => dump("i never run"));
           })).toThrow('1 active timer(s) are still in the queue.');
         });
+
+        it('should report no timers when there are none', async(() {
+          expect(isTimerQueueEmpty()).toBe(true);
+          expect(isNonPeriodicTimerQueueEmpty()).toBe(true);
+          expect(isPeriodicTimerQueueEmpty()).toBe(true);
+        }));
+
+        it('should report remaining non-periodic timers', async(() {
+          new Future(() => null);
+          expect(isTimerQueueEmpty()).toBe(false);
+          expect(isNonPeriodicTimerQueueEmpty()).toBe(false);
+          expect(isPeriodicTimerQueueEmpty()).toBe(true);
+          clockTick();
+        }));
+
+        it('should report remaining periodic timers', async(() {
+          var t = new Timer.periodic(new Duration(seconds: 1), (_) => null);
+          expect(isTimerQueueEmpty()).toBe(false);
+          expect(isNonPeriodicTimerQueueEmpty()).toBe(true);
+          expect(isPeriodicTimerQueueEmpty()).toBe(false);
+          t.cancel();
+        }));
       });
     });
   });