Replace navigation_and_routing with a new sample (#832)

* move snippets into old_snippets directory

* add new navigation_and_routing sample

* add copyright headers

* Apply #827 to old_snippets/ directory and upgrade them to null safety

* Code review comments

- Move Guard class into parser.dart
- Move usage of guards from Delegate to RouteInformationParser
- Rename delegate to SimpleRouterDelegate

* clean up imports

* refactor settings screen, fix bug

* avoid conflicting paths /books/new and /books/1 - rename to book/1

* dispose fields in _BookstoreState class

* remove /books path

This was causing problems

* add comment

* Change BookstoreAuthScope and BookstoreAuthScope to InheritedNotifier

* fix warnings

* Make the initial route configurable, set to '/signin'

* Enable deep linking

https://flutter.dev/docs/development/ui/navigation/deep-linking

* use path URL strategy on the web.

* remove TODO, add comment

navigation_and_routing/lib/src/screens/scaffold_body.dart

@@ -0,0 +1,62 @@
+// Copyright 2021, the Flutter project authors. Please see the AUTHORS file
+// 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.
+
+import 'package:flutter/material.dart';
+
+import '../routing.dart';
+import '../screens/settings.dart';
+import '../widgets/fade_transition_page.dart';
+import 'authors.dart';
+import 'books.dart';
+
+/// Displays the contents of the body of [BookstoreScaffold]
+class BookstoreScaffoldBody extends StatelessWidget {
+  static GlobalKey<NavigatorState> navigatorKey = GlobalKey<NavigatorState>();
+
+  const BookstoreScaffoldBody({
+    Key? key,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    var currentRoute = RouteStateScope.of(context)!.route;
+
+    // A nested Router isn't necessary because the back button behavior doesn't
+    // need to be customized.
+    return Navigator(
+      key: navigatorKey,
+      onPopPage: (route, dynamic result) => route.didPop(result),
+      pages: [
+        if (currentRoute.pathTemplate.startsWith('/authors'))
+          const FadeTransitionPage<void>(
+            key: ValueKey('authors'),
+            child: AuthorsScreen(),
+          )
+        else if (currentRoute.pathTemplate.startsWith('/settings'))
+          const FadeTransitionPage<void>(
+            key: ValueKey('settings'),
+            child: SettingsScreen(),
+          )
+        else if (currentRoute.pathTemplate.startsWith('/books') ||
+            currentRoute.pathTemplate == '/')
+          FadeTransitionPage<void>(
+            key: const ValueKey('books'),
+            child: BooksScreen(currentRoute: currentRoute),
+          )
+
+          // Avoid building a Navigator with an empty `pages` list when the
+          // RouteState is set to an unexpected path, such as /signin.
+          //
+          // Since RouteStateScope is an InheritedNotifier, any change to the
+          // route will result in a call to this build method, even though this
+          // widget isn't built when those routes are active.
+        else
+          FadeTransitionPage<void>(
+            key: const ValueKey('empty'),
+            child: Container(),
+          ),
+      ],
+    );
+  }
+}