Stop working around issue 8512.

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 bda216f35d8d092369d7235edf65d9d023dccca5..d60e41830881180de0134a678b7834d932c45a63 100644
--- a/pkg/scheduled_test/lib/src/mock_clock.dart
+++ b/pkg/scheduled_test/lib/src/mock_clock.dart
@@ -86,20 +86,14 @@ class _MockTimer implements Timer {
   /// The subscription to the [Clock.onTick] stream.
   StreamSubscription _subscription;
 
-  // TODO(nweiz): Remove this when issue 8512 is fixed.
-  var _cancelled = false;
-
   _MockTimer(Duration duration, this._callback)
       : _time = _clock.time + duration.inMilliseconds {
     _subscription = _clock.onTick.listen((time) {
-      if (_cancelled || time < _time) return;
+      if (time < _time) return;
       _subscription.cancel();
       _callback();
     });
   }
 
-  void cancel() {
-    _cancelled = true;
-    _subscription.cancel();
-  }
+  void cancel() => _subscription.cancel();
 }