Add hello_fn2.dart and make Text("Hello fn2") actually render something.

sky/examples/fn2/hello_fn2.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/framework/fn.dart';
+import 'package:sky/framework/fn2.dart';
 
 class HelloWorldApp extends App {
   UINode build() {
-    return new Text('Hello, world!');
+    return new Text('Hello, fn2!');
   }
 }
+
+void main() {
+  new HelloWorldApp();
+}