A widget to display before the current query in the AppBar.
Typically an IconButton configured with a BackButtonIcon that exits the search with close. One can also use an AnimatedIcon driven by transitionAnimation, which animates from e.g. a hamburger menu to the back button as the search overlay fades in.
Returns null if no widget should be shown.
- AppBar.leading, the intended use for the return value of this method.
Widget buildLeading(BuildContext context);