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/auth.dart

@@ -2,5 +2,47 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-export 'auth/auth.dart';
-export 'auth/auth_guard.dart';
+import 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+/// A mock authentication service
+class BookstoreAuth extends ChangeNotifier {
+  bool _signedIn = false;
+
+  bool get signedIn => _signedIn;
+
+  Future<void> signOut() async {
+    await Future<void>.delayed(const Duration(milliseconds: 200));
+    // Sign out.
+    _signedIn = false;
+    notifyListeners();
+  }
+
+  Future<bool> signIn(String username, String password) async {
+    await Future<void>.delayed(const Duration(milliseconds: 200));
+
+    // Sign in. Allow any password.
+    _signedIn = true;
+    notifyListeners();
+    return _signedIn;
+  }
+
+  @override
+  bool operator ==(Object other) =>
+      other is BookstoreAuth && other._signedIn == _signedIn;
+
+  @override
+  int get hashCode => _signedIn.hashCode;
+}
+
+class BookstoreAuthScope extends InheritedNotifier<BookstoreAuth> {
+  const BookstoreAuthScope({
+    required BookstoreAuth notifier,
+    required Widget child,
+    Key? key,
+  }) : super(key: key, notifier: notifier, child: child);
+
+  static BookstoreAuth of(BuildContext context) => context
+      .dependOnInheritedWidgetOfExactType<BookstoreAuthScope>()!
+      .notifier!;
+}