[Effen] add an assert that you don't try to re-add a defunct node

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 91 matching lines @@
     }
 
     if (node._willSync(oldNode)) {
       _traceSync(_SyncOperation.STATEFUL, node._key);
       oldNode._sync(node, host, insertBefore);
       node._defunct = true;
       assert(oldNode._root is RenderCSS);
       return oldNode;
     }
 
+    assert(!node._defunct);
     node._parent = this;
 
     if (oldNode == null) {
       _traceSync(_SyncOperation.INSERTION, node._key);
     } else {
       _traceSync(_SyncOperation.STATELESS, node._key);
     }
     node._sync(oldNode, host, insertBefore);
     if (oldNode != null)
       oldNode._defunct = true;
@@ skipping 799 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)]);
 }