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/test/library_test.dart

@@ -0,0 +1,22 @@
+// 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:bookstore/src/data/library.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('Library', () {
+    test('addBook', () {
+      final library = Library();
+      library.addBook('Left Hand of Darkness', 'Ursula K. Le Guin', true, true);
+      library.addBook('Too Like the Lightning', 'Ada Palmer', false, true);
+      library.addBook('Kindred', 'Octavia E. Butler', true, false);
+      library.addBook('The Lathe of Heaven', 'Ursula K. Le Guin', false, false);
+      expect(library.allAuthors.length, 3);
+      expect(library.allAuthors.first.books.length, 2);
+      expect(library.allBooks.length, 4);
+      expect(library.allBooks.first.author.name, startsWith('Ursula'));
+    });
+  });
+}