anchor.diff:  }
   
   class _EmptyMap<K, V> extends _ANodeBase<K, V> {
  -  _EmptyMap(_Owner owner) : super(owner, 0, false);
  +  _EmptyMap(_Owner owner) : super(owner, 0);
   
     V _get(K key, int hash, int depth) => _none;
   
  
positive.code:       return new _SubMap<K, V>.ensureOwner(this, owner, newMask, newarray, newSize);
      } else if (newarray.length == 1) {
        _ANodeBase<K, V> onlyValueLeft = newarray[0];
        return onlyValueLeft._isLeaf
            ? onlyValueLeft
            : new _SubMap<K, V>.ensureOwner(this, owner, newMask, newarray, newSize);
      } else {
  
negative.code: abstract class _ANodeBase<K, V> extends _NodeBase<K, V> {
    _Owner _owner;
  
    _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,
  