Clean up some Dart idioms in fn

sky/examples/fn/lib/component.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.
+
 part of fn;
 
 List<Component> _dirtyComponents = new List<Component>();
 bool _renderScheduled = false;
 
 void _renderDirtyComponents() {
   Stopwatch sw = new Stopwatch()..start();
 
   _dirtyComponents.sort((a, b) => a._order - b._order);
   for (var comp in _dirtyComponents) {
@@ skipping 12 matching lines @@
   if (!_renderScheduled) {
     _renderScheduled = true;
     new Future.microtask(_renderDirtyComponents);
   }
 }
 
 abstract class Component extends Node {
   bool _dirty = true; // components begin dirty because they haven't rendered.
   Node _rendered = null;
   bool _removed = false;
-  int _order;
+  final int _order;
   static int _currentOrder = 0;
   bool _stateful;
   static Component _currentlyRendering;
 
   Component({ Object key, bool stateful })
       : _stateful = stateful != null ? stateful : false,
         _order = _currentOrder + 1,
         super(key:key);
 
   void willUnmount() {}
@@ skipping 105 matching lines @@
 
     new Future.microtask(() {
       Stopwatch sw = new Stopwatch()..start();
       _sync(null, _host, null);
       assert(_root is sky.Node);
       sw.stop();
       print("Initial render: ${sw.elapsedMicroseconds} microseconds");
     });
   }
 }