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/data/library.dart

@@ -0,0 +1,41 @@
+// 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:collection/collection.dart';
+
+import 'author.dart';
+import 'book.dart';
+
+class Library {
+  final List<Book> allBooks = [];
+  final List<Author> allAuthors = [];
+
+  void addBook(String title, String authorName, bool isPopular, bool isNew) {
+    var author =
+        allAuthors.firstWhereOrNull((author) => author.name == authorName);
+    var book = Book(allBooks.length, title, isPopular, isNew);
+
+    if (author == null) {
+      author = Author(allAuthors.length, authorName, [book]);
+      allAuthors.add(author);
+    } else {
+      author.books.add(book);
+    }
+
+    book.author = author;
+    allBooks.add(book);
+  }
+
+  List<Book> get popularBooks {
+    return [
+      ...allBooks.where((book) => book.isPopular),
+    ];
+  }
+
+  List<Book> get newBooks {
+    return [
+      ...allBooks.where((book) => book.isNew),
+    ];
+  }
+}