anchor.diff:  
       // Produce notifications if needed
       _notifyChangeLength(len, value);
  -    if (_hasListObservers) {
  +    if (hasListObservers) {
         if (value < len) {
           _recordChange(new ListChangeRecord(this, value,
               removed: _list.getRange(value, len).toList()));
  
positive.code: 
    @reflectable void operator []=(int index, E value) {
      var oldValue = _list[index];
      if (_hasListObservers) {
        _recordChange(new ListChangeRecord(this, index, addedCount: 1,
            removed: [oldValue]));
      }
  
negative.code:     int len = _list.length;
  
      _notifyChangeLength(len, len - rangeLength);
      if (hasListObservers && rangeLength > 0) {
        _recordChange(new ListChangeRecord(this, start,
            removed: _list.getRange(start, end).toList()));
      }
  