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

runtime/observatory/lib/src/elements/helpers/rendering_scheduler.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 'package:observatory/src/elements/helpers/rendering_queue.dart';
 export 'package:observatory/src/elements/helpers/rendering_queue.dart';
 
 /// A generic renderable object.
@@ skipping 21 matching lines @@
   bool _dirty = false;
   bool _renderingScheduled = false;
   bool _notificationScheduled = false;
 
   /// Element managed by this scheduler.
   final T element;
 
   /// Queue used for rendering operations.
   final RenderingQueue queue;
 
+  final List<Future> _wait = <Future>[];
+
   /// Does the element need a new rendering cycle.
   bool get isDirty => _dirty;
 
   /// Is the scheduler enabled.
   bool get isEnabled => _enabled;
 
   final StreamController<RenderedEvent<T>> _onRendered =
       new StreamController<RenderedEvent<T>>.broadcast();
   Stream<RenderedEvent<T>> get onRendered => _onRendered.stream;
 
@@ skipping 51 matching lines @@
     if (!_enabled) return;
     queue.enqueue(this);
     _renderingScheduled = true;
   }
 
   /// Renders the element (if the scheduler is enabled).
   /// It will clear the dirty flag.
   void render() {
     if (!_enabled) return;
     _dirty = false;
+    _wait.clear();
     element.render();
     _renderingScheduled = false;
     scheduleNotification();
     if (_dirty) scheduleRendering();
   }
 
   /// Schedules a notification.
   void scheduleNotification() {
     if (_notificationScheduled) return;
     _notify();
     _notificationScheduled = true;
   }
 
+  void waitFor(Iterable<Future> it) {
+    _wait.addAll(it);
+  }
+
   Future _notify() async {
+    await Future.wait(_wait);
+    _wait.clear();
     _onRendered.add(new RenderedEvent<T>(element, _dirty));
     _notificationScheduled = false;
   }
 }