anchor.diff:    _runTimer(_FakeTimer timer) {
       assert(timer.isActive);
       _advanceTo(timer._nextCall);
  -    if(timer._isPeriodic) {
  +    if (timer._isPeriodic) {
         timer._callback(timer);
         timer._nextCall = timer._nextCall.add(timer._duration);
       } else {
  
positive.code:       });
  
    _advanceTo(DateTime to) {
      if(to.millisecondsSinceEpoch > _now.millisecondsSinceEpoch) {
        _now = to;
      }
    }
  
negative.code:   DateTime now() => _now;
  
    Future advance(Duration duration) {
      if (duration.inMicroseconds < 0) {
        return new Future.error(
            new ArgumentError('Cannot call advance with negative duration'));
      }
      if (_advancingTo != null) {
        return new Future.error(
            new StateError('Cannot advance until previous advance is complete.'));
      }
  