anchor.diff:    }
   
   
  -  remove() {
  +  void remove() {
       _group._recordRemove(this);
     }
   
  -  toString() => '${_MODE_NAMES[_mode]}[$field]';
  +  String toString() => '${_MODE_NAMES[_mode]}[$field]';
   }
   
   final Object _INITIAL_ = new Object();
  
positive.code:         // is the same. We save the value so that next time identity will pass
          currentValue = current;
        } else if (last is num && last.isNaN && current is num && current.isNaN) {
          // we need this for JavaScript since in JS NaN !== NaN.
        } else {
          previousValue = last;
          currentValue = current;
  
negative.code:     return record;
    }
  
    void clear() {
      map.clear();
    }
  }
  