Refactor fn2.dart, since it breached our 1000-line threshold.

sky/sdk/lib/framework/scheduler.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 'dart:async';
 import 'dart:sky' as sky;
 
 typedef void Callback(double timeStamp);
 
 bool _haveScheduledVisualUpdate = false;
 int _nextCallbackId = 1;
 
 final List<Callback> _persistentCallbacks = new List<Callback>();
 Map<int, Callback> _transientCallbacks = new Map<int, Callback>();
 
@@ skipping 36 matching lines @@
   _transientCallbacks.remove(id);
 }
 
 void ensureVisualUpdate() {
   assert(sky.window == null);
   if (_haveScheduledVisualUpdate)
     return;
   sky.view.scheduleFrame();
   _haveScheduledVisualUpdate = true;
 }