anchor.diff:    }
   
     @override
  -  Iterator<E> get iterator => new _ImmutableListIterator<E>(_root,
  +  Iterator<E> get iterator => new _AvlImmutableListIterator<E>(_root,
         startIndex: _start, count: _end - _start);
   }
  
positive.code:       if (startIndex >= _length) startIndex = _length - 1;
      }
  
      final iterator = new _ImmutableListIterator(this,
          startIndex: startIndex, reversed: true);
      var index = startIndex;
      while (iterator.moveNext()) {
  
negative.code:   /// The version of the builder (when applicable) that is being iterated.
    int _iteratingBuilderVersion;
  
    _AvlImmutableListIterator(_AvlNode root,
        {_AvlImmutableListBuilder builder,
        int startIndex: -1,
        int count: -1,
  