Controls whether we should try to imply the leading widget if null.
Specifically this navigation bar will:
- Show a 'Close' button if the current route is a
- Show a back chevron with previousPageTitle if previousPageTitle is not null.
- Show a back chevron with the previous route's
titleif the current route is a CupertinoPageRoute and the previous route is also a CupertinoPageRoute.
This value cannot be null.
final bool automaticallyImplyLeading;