dispose method

Implementation

@override
void dispose() {
  super.dispose();

  // Do not forget to clear reference variables, or it will cause memory leaks!
  _flexLineBoxMetrics.clear();
  _childrenIntrinsicMainSizes = Expando<double>('childrenIntrinsicMainSizes');
  _childrenOldConstraints = Expando<BoxConstraints>('childrenOldConstraints');
}