buildWithChild method
A build method that receives an extra child parameter.
This method may be called with a child different from the parameter
passed to the constructor of SingleChildStatelessModifier.
It may also be called again with a different child, without this widget
being recreated.
Implementation
@override
Widget buildWithChild(BuildContext context, Widget? child) {
return Flexible(
key: modifierKey,
fit: fit,
flex: flex,
child: child!,
);
}