anchor.diff:        if (startIndex >= _length) startIndex = _length - 1;
       }
   
  -    final iterator = new _ImmutableListIterator(this,
  +    final iterator = new _AvlImmutableListIterator(this,
           startIndex: startIndex, reversed: true);
       var index = startIndex;
       while (iterator.moveNext()) {
  
positive.code:   bool get isEmpty => _root.isEmpty;
  
    @override
    Iterator<E> get iterator => new _ImmutableListIterator(_root);
  
    @override
    E get last {
  
negative.code:   }
  
    @override
    Iterator<E> get iterator => new _AvlImmutableListIterator<E>(_root,
        startIndex: _start, count: _end - _start);
  }
  