automaticallyImplyLeading property

Controls whether we should try to imply the leading widget if null.

If true and leading is null, automatically try to deduce what the leading widget should be. If leading widget is not null, this parameter has no effect.

Specifically this navigation bar will:

1. Show a 'Close' button if the current route is a fullscreenDialog.
2. Show a back chevron with previousPageTitle if previousPageTitle is not null.
3. Show a back chevron with the previous route's title if the current route is a CupertinoPageRoute and the previous route is also a CupertinoPageRoute.

This value cannot be null.