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/anonymous_routes.dart

@@ -0,0 +1,61 @@
+// 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 use [Navigator] APIs to push and pop an anonymous
+/// route. In this case, it is an instance of [MaterialPageRoute].
+library anonymous_routes;
+
+import 'package:flutter/material.dart';
+
+void main() {
+  runApp(Nav2App());
+}
+
+class Nav2App extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      home: HomeScreen(),
+    );
+  }
+}
+
+class HomeScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: TextButton(
+          child: Text('View Details'),
+          onPressed: () {
+            Navigator.push(
+              context,
+              MaterialPageRoute(builder: (context) {
+                return DetailScreen();
+              }),
+            );
+          },
+        ),
+      ),
+    );
+  }
+}
+
+class DetailScreen extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(),
+      body: Center(
+        child: TextButton(
+          child: Text('Pop!'),
+          onPressed: () {
+            Navigator.pop(context);
+          },
+        ),
+      ),
+    );
+  }
+}