Stop working around issue 8512.

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 68 matching lines @@
 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;
 
-  // 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();
 }