Fixed synchronization problem in FF and Safari for virtual-tree test

runtime/observatory/lib/src/elements/containers/virtual_tree.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. 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:html';
 import 'dart:collection';
 import 'dart:math' as Math;
 import 'package:observatory/src/elements/containers/virtual_collection.dart';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
@@ skipping 159 matching lines @@
               .forEach((c) => toDo.addFirst(c));
           final depth = depths[items.length - 1];
           depths.insertAll(
               items.length, new List.filled(children.length, depth + 1));
         }
       }
     }
 
     _depths = depths;
     _collection.items = items;
+
+    _r.waitFor([_collection.onRendered.first]);
   }
 }