[Effen] Add _syncChild assert and cleanup trace output

sky/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 104 matching lines @@
 
   void _removeChild(Node node) {
     _trace('_sync(remove) ${node._key}');
     node._remove();
   }
 
   // Returns the child which should be retained as the child of this node.
   Node _syncChild(Node node, Node oldNode, sky.ParentNode host,
       sky.Node insertBefore) {
 
+    assert(oldNode == null || node._key == oldNode._key);
+
     if (node == oldNode) {
       _trace('_sync(identical) ${node._key}');
       return node; // Nothing to do. Subtrees must be identical.
     }
 
     // TODO(rafaelw): This eagerly removes the old DOM. It may be that a
     // new component was built that could re-use some of it. Consider
     // syncing the new VDOM against the old one.
     if (oldNode != null && node._key != oldNode._key) {
       _trace('_sync(remove) ${node._key}');
       oldNode._remove();
     }
 
     if (node._willSync(oldNode)) {
-      _trace('_sync(statefull) ${node._key} -> ${oldNode._key}');
+      _trace('_sync(statefull) ${node._key}');
       oldNode._sync(node, host, insertBefore);
       node._defunct = true;
       assert(oldNode._root is sky.Node);
       return oldNode;
     }
 
     node._parent = this;
 
     if (oldNode == null) {
       _trace('_sync(insert) ${node._key}');
     } else {
-      _trace('_sync(stateless) ${node._key} <- ${oldNode._key}');
+      _trace('_sync(stateless) ${node._key}');
     }
     node._sync(oldNode, host, insertBefore);
     if (oldNode != null)
       oldNode._defunct = true;
 
     assert(node._root is sky.Node);
     return node;
   }
 }
 
@@ skipping 643 matching lines @@
 
 abstract class App extends Component {
   sky.Node _host;
 
   App() : super(stateful: true) {
     _host = sky.document.createElement('div');
     sky.document.appendChild(_host);
     _scheduleComponentForRender(this);
   }
 }