anchor.diff:  
     bool add(E object) => super._add(object);
   
  -  bool contains(Object object) {
  +  bool contains(Object? object) {
       if (!_validKey(object)) return false;
       return super._contains(object);
     }
   
  -  E lookup(Object object) {
  +  E? lookup(Object? object) {
       if (!_validKey(object)) return null;
       return super._lookup(object);
     }
   
  -  bool remove(Object object) {
  +  bool remove(Object? object) {
       if (!_validKey(object)) return false;
       return super._remove(object);
     }
  
positive.code:     return true;
    }
  
    bool _removeHashTableEntry(var table, Object element) {
      if (table == null) return false;
      _LinkedHashSetCell cell = _getTableEntry(table, element);
      if (cell == null) return false;
  
negative.code:     return true;
    }
  
    bool _removeHashTableEntry(var table, Object? element) {
      if (table != null && _hasTableEntry(table, element)) {
        _deleteTableEntry(table, element);
        _length--;
  