anchor.diff:  
          Future parse(_) {
   
  -         _cancel.check();
  +         _checkCancel();
            PackedObject object = _matchObjectAtOffset(_offset);
            object.crc = getCrc32(data.sublist(object.offset, _offset));
   
  
positive.code:        }
  
         Future expandDeltified(PackedObject obj) {
           _cancel.check();
           return expandDeltifiedObject(obj).then((PackedObject deltifiedObj) {
             deltifiedObj.data = null;
             // TODO(grv) : add progress.
  
negative.code: 
    void _advance(int length) {
      _offset += length;
    }
  
    bool _checkCancel() {
      if (_cancel != null) {
        return _cancel.check();
      } else {
        return true;
      }
    }
  
    void _matchPrefix() {
  