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