Rename Navigator 2.0 to Router (#712)

* Rename Navigator 2.0 to Router

This aligns better with the documentation for this new API, and doesn't
imply that apps need to be upgraded.

* introduction -> navigator

navigation_and_routing/lib/navigator/on_generate_route.dart

@@ -0,0 +1,98 @@
+// 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 handle arbitrary named routes using the `onGenerateRoute`
+/// callback defined in the `MaterialApp` constructor.
+library on_generate_route;
+
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(Nav2App());
+}
+
+class Nav2App extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      onGenerateRoute: (settings) {
+        // Handle '/'
+        if (settings.name == '/') {
+          return MaterialPageRoute(builder: (context) => HomeScreen());
+        }
+
+        // Handle '/details/:id'
+        var uri = Uri.parse(settings.name);
+        if (uri.pathSegments.length == 2 &&
+            uri.pathSegments.first == 'details') {
+          var id = uri.pathSegments[1];
+          return MaterialPageRoute(builder: (context) => DetailScreen(id: id));
+        }
+
+        return MaterialPageRoute(builder: (context) => UnknownScreen());
+      },
+    );
+  }
+}
+
+class HomeScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: TextButton(
+          child: Text('View Details'),
+          onPressed: () {
+            Navigator.pushNamed(
+              context,
+              '/details/1',
+            );
+          },
+        ),
+      ),
+    );
+  }
+}
+
+class DetailScreen extends StatelessWidget {
+  final String id;
+
+  DetailScreen({
+    this.id,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: Column(
+          mainAxisAlignment: MainAxisAlignment.center,
+          children: [
+            Text('Viewing details for item $id'),
+            TextButton(
+              child: Text('Pop!'),
+              onPressed: () {
+                Navigator.pop(context);
+              },
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}
+
+class UnknownScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: Text('404!'),
+      ),
+    );
+  }
+}