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/old_snippets/router/pages.dart

@@ -0,0 +1,147 @@
+// Copyright 2020, 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.
+
+/// Shows how to define a list of [Page] objects on Navigator declaratively.
+library nav2_pages;
+
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(const BooksApp());
+}
+
+class Book {
+  final String title;
+  final String author;
+
+  Book(this.title, this.author);
+}
+
+class BooksApp extends StatefulWidget {
+  const BooksApp({Key? key}) : super(key: key);
+
+  @override
+  State<StatefulWidget> createState() => _BooksAppState();
+}
+
+class _BooksAppState extends State<BooksApp> {
+  Book? _selectedBook;
+
+  final List<Book> books = [
+    Book('Left Hand of Darkness', 'Ursula K. Le Guin'),
+    Book('Too Like the Lightning', 'Ada Palmer'),
+    Book('Kindred', 'Octavia E. Butler'),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Books App',
+      home: Navigator(
+        pages: [
+          MaterialPage<void>(
+            key: const ValueKey('BooksListPage'),
+            child: BooksListScreen(
+              books: books,
+              onTapped: _handleBookTapped,
+            ),
+          ),
+          if (_selectedBook != null) BookDetailsPage(book: _selectedBook)
+        ],
+        onPopPage: (route, dynamic result) {
+          if (!route.didPop(result)) {
+            return false;
+          }
+
+          // Update the list of pages by setting _selectedBook to null
+          setState(() {
+            _selectedBook = null;
+          });
+
+          return true;
+        },
+      ),
+    );
+  }
+
+  void _handleBookTapped(Book book) {
+    setState(() {
+      _selectedBook = book;
+    });
+  }
+}
+
+class BookDetailsPage extends Page<void> {
+  final Book? book;
+
+  BookDetailsPage({
+    this.book,
+  }) : super(key: ValueKey(book));
+
+  @override
+  Route createRoute(BuildContext context) {
+    return MaterialPageRoute<void>(
+      settings: this,
+      builder: (context) {
+        return BookDetailsScreen(book: book);
+      },
+    );
+  }
+}
+
+class BooksListScreen extends StatelessWidget {
+  final List<Book> books;
+  final ValueChanged<Book> onTapped;
+
+  const BooksListScreen({
+    required this.books,
+    required this.onTapped,
+    Key? key,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: ListView(
+        children: [
+          for (var book in books)
+            ListTile(
+              title: Text(book.title),
+              subtitle: Text(book.author),
+              onTap: () => onTapped(book),
+            )
+        ],
+      ),
+    );
+  }
+}
+
+class BookDetailsScreen extends StatelessWidget {
+  final Book? book;
+
+  const BookDetailsScreen({
+    @required this.book,
+    Key? key,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Padding(
+        padding: const EdgeInsets.all(8.0),
+        child: Column(
+          crossAxisAlignment: CrossAxisAlignment.start,
+          children: [
+            if (book != null) ...[
+              Text(book!.title, style: Theme.of(context).textTheme.headline6),
+              Text(book!.author, style: Theme.of(context).textTheme.subtitle1),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}