Implement a simple checkbox in Sky’s fn2 components library

R=abarth@chromium.org, abarth

Review URL: https://codereview.chromium.org/1161813005

examples/widgets/main.sky

+#!mojo mojo:sky_viewer
+<!--
+// 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.
+-->
+<sky>
+<import src="/sky/framework/debug/shake-to-reload.sky" />
+<script>
+import 'widgets_app.dart';
+void main() {
+  new WidgetsApp();
+}
+</script>
+</sky>