anchor.diff:    @override
     bool containsKey(Object key) {
       if (key == null) return _hasNull;
  -    return (_root != null)
  -        ? _root.find(0, key.hashCode, key as K, _notFound as V) != _notFound
  +    return _root != null
  +        ? !identical(
  +            _root.find(0, key.hashCode, key as K, _notFound as V), _notFound)
           : false;
     }
   
  
positive.code:     _leafFlag.value = null;
      Node<K, V> n = (_root == null ? new BitmapIndexedNode<K, V>.empty() : _root)
          .addM(0, key.hashCode, key, value, _leafFlag);
      if (n != _root) _root = n;
      if (_leafFlag.value != null) _length--;
    }
  
  
negative.code:     }
      if (_root == null) return this;
      Node<K, V> newRoot = _root.remove(0, key.hashCode, key as K);
      if (identical(newRoot, _root)) return this;
      return new HamtImmutableMap._internal(
          length - 1, newRoot, _hasNull, _nullValue);
    }
  