anchor.diff:      _list.setRange(index + 1, length, this, index);
   
       _notifyChangeLength(_list.length - 1, _list.length);
  -    if (_hasListObservers) {
  +    if (hasListObservers) {
         _recordChange(new ListChangeRecord(this, index, addedCount: 1));
       }
       _list[index] = element;
  
positive.code:     _notifyChangeLength(len, _list.length);
  
      int added = _list.length - len;
      if (_hasListObservers && added > 0) {
        _recordChange(new ListChangeRecord(this, len, addedCount: added));
      }
    }
  
negative.code:   void add(E value) {
      int len = _list.length;
      _notifyChangeLength(len, len + 1);
      if (hasListObservers) {
        _recordChange(new ListChangeRecord(this, len, addedCount: 1));
      }
  
  