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:   }
  
    void advanceSync(Duration duration) {
      if (duration.inMicroseconds < 0) {
        throw new ArgumentError('Cannot call advance with negative duration');
      }
      _now = _now.add(duration);
    }
  
    Zone get zone {
      if (_zone == null) {
        _zone = Zone.current.fork(specification: _zoneSpec);
      }
      return _zone;
  