Implement a simple checkbox in Sky’s fn2 components library

sky/examples/widgets/widgets_app.dart

Index: sky/examples/widgets/widgets_app.dart
diff --git a/sky/examples/widgets/widgets_app.dart b/sky/examples/widgets/widgets_app.dart
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+// 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/framework/fn.dart';
+import 'package:sky/framework/components/button.dart';
+import 'package:sky/framework/components/input.dart';
+import 'package:sky/framework/components/popup_menu.dart';
+
+class WidgetsApp extends App {
+  static final Style _menuStyle = new Style('''
+    position: absolute;
+    top: 200px;
+    left: 200px;''');
+
+  PopupMenuController controller;
+
+  UINode build() {
+    return new Container(
+      children: [
+        new Button(key: 'Go', content: new Text('Go'), level: 1),
+        new Button(key: 'Back', content: new Text('Back'), level: 3),
+        new Input()
+
+        // PopupMenu requires a PopupMenuController and should be wired up
+        // to a button to create the controller only when the menu is open, etc.
+        // new Container(
+        //   style: _menuStyle,
+        //   children: [
+        //     new PopupMenu(
+        //       controller: controller,
+        //       items: [
+        //         [new Text('People & options')],
+        //         [new Text('New group conversation')],
+        //         [new Text('Turn history off')],
+        //         [new Text('Archive')],
+        //         [new Text('Delete')],
+        //         [new Text('Un-merge SMS')],
+        //         [new Text('Help & feeback')],
+        //       ],
+        //       level: 4),
+        //     ]
+        //   )
+        // ]
+      );
+  }
+}