Restore "Speedup Observatory elements first time rendering"

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 14 matching lines @@
     assert(otherRenderScheduled != null);
   }
 }
 
 /// Scheduler for rendering operations.
 class RenderingScheduler<T extends Renderable> implements RenderingTask {
   bool _enabled = false;
   bool _dirty = false;
   bool _renderingScheduled = false;
   bool _notificationScheduled = false;
+  bool _waitForBarrier = 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.
@@ skipping 51 matching lines @@
       dirty();
     else
       scheduleNotification();
     return newValue;
   }
 
   /// Schedules a new rendering phase.
   void scheduleRendering() {
     if (_renderingScheduled) return;
     if (!_enabled) return;
-    queue.enqueue(this);
+    queue.enqueue(this, waitForBarrier: _waitForBarrier);
+    _waitForBarrier = true;
     _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();
@@ skipping 13 matching lines @@
     _wait.addAll(it);
   }
 
   Future _notify() async {
     await Future.wait(_wait);
     _wait.clear();
     _onRendered.add(new RenderedEvent<T>(element, _dirty));
     _notificationScheduled = false;
   }
 }