leading property
A widget to display before the title.
Typically the leading widget is an Icon or an IconButton.
Becomes the leading component of the NavigationToolBar built
by this widget. The leading widget's width and height are constrained to
be no bigger than toolbar's height, which is kToolbarHeight.
If this is null and automaticallyImplyLeading is set to true, the EmergentAppBar will imply an appropriate widget. For example, if the EmergentAppBar is in a Scaffold that also has a Drawer, the Scaffold will fill this widget with an IconButton that opens the drawer (using Icons.menu). If there's no Drawer and the parent Navigator can go back, the EmergentAppBar will use a EmergentBackButton that calls Navigator.maybePop.
Implementation
final Widget? leading;