anchor.diff:  
     _scheduleTimer(Zone self, ZoneDelegate parent, Zone zone) {
   
  -    if(!_waitingForTimer && _advancingTo != null) {
  +    if (!_waitingForTimer && _advancingTo != null) {
         var next = _getNextTimer();
         var completeTimer = next != null ?
             self.bindCallback(() => _runTimer(next), runGuarded: true) :
  
positive.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.'));
      }
  
negative.code:   _runTimer(_FakeTimer timer) {
      assert(timer.isActive);
      _advanceTo(timer._nextCall);
      if (timer._isPeriodic) {
        timer._callback(timer);
        timer._nextCall = timer._nextCall.add(timer._duration);
      } else {
  