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/widgets/fade_transition_page.dart

@@ -0,0 +1,52 @@
+// 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';
+
+class FadeTransitionPage<T> extends Page<T> {
+  final Widget child;
+
+  const FadeTransitionPage({LocalKey? key, required this.child})
+      : super(key: key);
+
+  @override
+  Route<T> createRoute(BuildContext context) {
+    return PageBasedFadeTransitionRoute<T>(this);
+  }
+}
+
+class PageBasedFadeTransitionRoute<T> extends PageRoute<T> {
+  PageBasedFadeTransitionRoute(Page page)
+      : super(
+          settings: page,
+        );
+
+  @override
+  Color? get barrierColor => null;
+
+  @override
+  String? get barrierLabel => null;
+
+  @override
+  Duration get transitionDuration => const Duration(milliseconds: 300);
+
+  @override
+  bool get maintainState => true;
+
+  @override
+  Widget buildPage(BuildContext context, Animation<double> animation,
+      Animation<double> secondaryAnimation) {
+    var curveTween = CurveTween(curve: Curves.easeIn);
+    return FadeTransition(
+      opacity: animation.drive(curveTween),
+      child: (settings as FadeTransitionPage).child,
+    );
+  }
+
+  @override
+  Widget buildTransitions(BuildContext context, Animation<double> animation,
+      Animation<double> secondaryAnimation, Widget child) {
+    return child;
+  }
+}