navigation_and_routing: a bunch of cleanup (#886)

Fail early on nullable context objects - no code paths allow null
Eliminate superfluous private function
Use a function over an abstract class with one method

navigation_and_routing/lib/src/screens/scaffold_body.dart

@@ -21,7 +21,7 @@ class BookstoreScaffoldBody extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    var currentRoute = RouteStateScope.of(context)!.route;
+    var currentRoute = RouteStateScope.of(context).route;
 
     // A nested Router isn't necessary because the back button behavior doesn't
     // need to be customized.