[Effen] Try to catch more misuse of fn by asserting that UINode nodes are only built when we're upd…

sky/sdk/lib/framework/fn.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.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
@@ skipping 14 matching lines @@
  */
 abstract class UINode {
   String _key;
   UINode _parent;
   UINode get parent => _parent;
   RenderCSS _root;
   bool _defunct = false;
 
   UINode({ Object key }) {
     _key = key == null ? "$runtimeType" : "$runtimeType-$key";
+    assert(this is App || _inRenderDirtyComponents); // you should not build the UI tree ahead of time, build it only during build()
   }
 
   // Subclasses which implements Nodes that become stateful may return true
   // if the |old| node has become stateful and should be retained.
   bool _willSync(UINode old) => false;
 
   bool get interchangeable => false; // if true, then keys can be duplicated
 
   void _sync(UINode old, RenderCSSContainer host, RenderCSS insertBefore);
 
@@ skipping 877 matching lines @@
     _sync(null, _host, _root);
   }
 }
 
 class Text extends Component {
   Text(this.data) : super(key: '*text*');
   final String data;
   bool get interchangeable => true;
   UINode build() => new Paragraph(children: [new TextFragment(data)]);
 }