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/settings.dart

@@ -0,0 +1,103 @@
+// 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 'package:url_launcher/link.dart';
+
+import '../auth/auth.dart';
+
+class SettingsScreen extends StatefulWidget {
+  const SettingsScreen({Key? key}) : super(key: key);
+
+  @override
+  _SettingsScreenState createState() => _SettingsScreenState();
+}
+
+class _SettingsScreenState extends State<SettingsScreen> {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      body: SafeArea(
+        child: SingleChildScrollView(
+          child: Align(
+            alignment: Alignment.topCenter,
+            child: ConstrainedBox(
+              constraints: const BoxConstraints(maxWidth: 400),
+              child: const Card(
+                child: Padding(
+                  padding: EdgeInsets.symmetric(vertical: 18, horizontal: 12),
+                  child: SettingsContent(),
+                ),
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
+
+class SettingsContent extends StatelessWidget {
+  const SettingsContent({
+    Key? key,
+  }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    return Column(
+      children: [
+        ...[
+          Text(
+            'Settings',
+            style: Theme.of(context).textTheme.headline4,
+          ),
+          ElevatedButton(
+            onPressed: () {
+              BookstoreAuthScope.of(context)!.signOut();
+            },
+            child: const Text('Sign out'),
+          ),
+          Link(
+            uri: Uri.parse('/book/0'),
+            builder: (context, followLink) {
+              return TextButton(
+                child: const Text('Go directly to /book/0'),
+                onPressed: followLink,
+              );
+            },
+          ),
+          Link(
+            uri: Uri.parse('/author/0'),
+            builder: (context, followLink) {
+              return TextButton(
+                child: const Text('Go directly to /author/0'),
+                onPressed: followLink,
+              );
+            },
+          ),
+        ].map((w) => Padding(padding: const EdgeInsets.all(8), child: w)),
+        TextButton(
+          onPressed: () => showDialog<String>(
+            context: context,
+            builder: (context) => AlertDialog(
+              title: const Text('Alert!'),
+              content: const Text('The alert description goes here.'),
+              actions: <Widget>[
+                TextButton(
+                  onPressed: () => Navigator.pop(context, 'Cancel'),
+                  child: const Text('Cancel'),
+                ),
+                TextButton(
+                  onPressed: () => Navigator.pop(context, 'OK'),
+                  child: const Text('OK'),
+                ),
+              ],
+            ),
+          ),
+          child: const Text('Show Dialog'),
+        )
+      ],
+    );
+  }
+}