anchor.diff:  abstract class _ANodeBase<K, V> extends _NodeBase<K, V> {
     _Owner _owner;
   
  -  _ANodeBase(this._owner, length, this._isLeaf) : super(length);
  -
  -  final bool _isLeaf;
  +  _ANodeBase(this._owner, length) : super(length);
   
     V _get(K key, int hash, int depth);
     _NodeBase<K, V> _insertWith(_Owner owner, LinkedList<Pair<K, V>> keyValues, int size,
  
positive.code:           assert(_array.length == 2);
            assert(index == 0 || index == 1);
            _ANodeBase<K, V> onlyValueLeft = _array[1 - index];
            return onlyValueLeft._isLeaf
                ? onlyValueLeft
                : new _SubMap.ensureOwner(this, owner, _bitmap ^ mask,
                              <_ANodeBase<K, V>>[onlyValueLeft],
                              length + delta);
          }
        } else if (newm._isLeaf){
          if (_array.length == 1) {
            return newm;
          } else {
  
negative.code:   int _hash;
    LinkedList<Pair<K, V>> _pairs;
  
    _Leaf.abc(_Owner owner, this._hash, pairs, int size) : super(owner, size) {
      this._pairs = pairs;
      assert(size == pairs.length);
    }
  