anchor.diff:    _Hasher<E> _hasher;
     _Predicate _validKey;
     _LinkedCustomHashSet(
  -      this._equality, this._hasher, bool validKey(potentialKey))
  +      this._equality, this._hasher, bool validKey(potentialKey)?)
         : _validKey = (validKey != null) ? validKey : ((x) => x is E);
   
     Set<E> _newSet() =>
  
positive.code:       }
      }
      assert(index == _length);
      return _keys = result;
    }
  
    void _addHashTableEntry(var table, K key, V value) {
  
negative.code:   _Equality<E> _equality;
    _Hasher<E> _hasher;
    _Predicate _validKey;
    _CustomHashSet(this._equality, this._hasher, bool validKey(potentialKey)?)
        : _validKey = (validKey != null) ? validKey : ((x) => x is E);
  
    Set<E> _newSet() => new _CustomHashSet<E>(_equality, _hasher, _validKey);
  