Avoid to schedule unnecessary empty rendering cicles

runtime/observatory/lib/src/elements/helpers/rendering_queue.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:html';
 import 'dart:collection';
 import 'dart:async';
 
 /// A generic rendering task that can be scheduled.
 abstract class RenderingTask {
@@ skipping 48 matching lines @@
   void enqueue(RenderingTask r) {
     assert(r != null);
     // If no task are in the queue there is no rendering phase scheduled.
     if (isEmpty) _render();
     _queue.addLast(r);
   }
 
   Future _render() async {
     await _barrier.next;
     while (_queue.isNotEmpty) {
-      _queue.removeFirst().render();
+      _queue.first.render();
+      _queue.removeFirst();
     }
   }
 }