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: 
    bool add(E element) => super._add(element);
  
    bool contains(Object object) {
      if (!_validKey(object)) return false;
      return super._contains(object);
    }
  
    E lookup(Object object) {
      if (!_validKey(object)) return null;
      return super._lookup(object);
    }
  
    bool remove(Object object) {
      if (!_validKey(object)) return false;
      return super._remove(object);
    }
  
    bool containsAll(Iterable<Object> elements) {
      for (Object element in elements) {
        if (!_validKey(element) || !this.contains(element)) return false;
      }
      return true;
    }
  
    void removeAll(Iterable<Object> elements) {
      for (Object element in elements) {
        if (_validKey(element)) {
          super._remove(element);
        }
  
negative.code:   bool get isEmpty => _length == 0;
    bool get isNotEmpty => !isEmpty;
  
    bool contains(Object? object) {
      if (_isStringElement(object)) {
        var strings = _strings;
        if (strings == null) return false;
  