anchor.diff:    /// The version of the builder (when applicable) that is being iterated.
     int _iteratingBuilderVersion;
   
  -  _ImmutableListIterator(_AvlNode root,
  +  _AvlImmutableListIterator(_AvlNode root,
         {_AvlImmutableListBuilder builder,
         int startIndex: -1,
         int count: -1,
  
positive.code:   }
  
    Iterator<E> _iterator(ImmutableListBuilder builder) =>
        new _ImmutableListIterator(this, builder: builder);
  
    /// Creates a node mutation, either by mutating this node (if not yet frozen)
    /// or by creating a clone of this node with the described changes.
  
negative.code: 
    @override
    Iterator<E> get iterator =>
        new _AvlImmutableListIterator<E>(_root, reversed: true);
  }
  
  class _SubListIterable<E> extends IterableBase<E> {
  