anchor.diff:    String get rawValue => _rawValue;
   
     @override
  -  void _updateValue() {}
  +  void onUpdate() {}
   
     @override
     bool _anyControlsHaveStatus(String status) => false;
  
positive.code:     return VALID;
    }
  
    void _updateValue();
    bool _anyControlsHaveStatus(String status);
  }
  
  
negative.code:   num get length => controls.length;
  
    @override
    void onUpdate() {
      _value = controls.map((control) => control.value).toList();
    }
  
  