read-only
A widget to display before the title.
If this field is null and this app bar is used in a Scaffold, the Scaffold will fill this field with an appropriate widget. For example, if the Scaffold also has a Drawer, the Scaffold will fill this widget with an IconButton that opens the drawer. If there's no Drawer and the parent Navigator can go back, the Scaffold will fill this field with an IconButton that calls Navigator.pop.