Simple support for routing in Sky

R=abarth@chromium.org, abarth, hixie

Review URL: https://codereview.chromium.org/1186273002.

examples/raw/navigation.dart

+// Copyright 2015 The Chromium Authors. 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:sky/widgets/basic.dart';
+import 'package:sky/widgets/navigator.dart';
+import 'package:sky/widgets/raised_button.dart';
+
+List<Route> routes = [
+  new Route(
+    name: 'safety',
+    builder: (navigator) => new RaisedButton(
+      child: new Text('PRESS FORWARD'),
+      onPressed: () => navigator.pushNamedRoute('adventure')
+    )
+  ),
+  new Route(
+    name: 'adventure',
+    builder: (navigator) => new RaisedButton(
+      child: new Text('NO WAIT! GO BACK!'),
+      onPressed: () => navigator.pushRoute(routes[0])
+    )
+  )
+];
+
+class NavigationExampleApp extends App {
+  UINode build() {
+    return new Navigator(routes: routes);
+  }
+}
+
+void main() {
+  App app = new NavigationExampleApp();
+}