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:     }
    }
  
    Iterable get iterable => _iterable;
  
    bool _check(Iterable collection) {
      _reset();
      ItemRecord record = _collectionHead;
      bool maybeDirty = false;
  