A widget to display before the title.
If this is null and automaticallyImplyLeading is set to true, the AppBar will imply an appropriate widget. For example, if the AppBar is in a Scaffold that 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 AppBar will use a BackButton that calls Navigator.maybePop.
final Widget leading