Restore "Speedup Observatory elements first time rendering"

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 38 matching lines @@
   RenderingQueue() : this.fromBarrier(new NextAnimationFrameBarrier());
 
   /// Creates a RenderingQueue with a custom synchronization barrier.
   RenderingQueue.fromBarrier(this._barrier) {
     assert(this._barrier != null);
   }
 
   /// Add a task to the queue.
   /// If the current rendering phase is running it will be executed during this
   /// rendering cycle, otherwise it will be queued for the next one.
-  void enqueue(RenderingTask r) {
+  void enqueue(RenderingTask r, {bool waitForBarrier: true}) {
     assert(r != null);
+    final wasEmpty = _queue.isEmpty;
+    _queue.addLast(r);
     // If no task are in the queue there is no rendering phase scheduled.
-    if (isEmpty) _render();
-    _queue.addLast(r);
+    if (wasEmpty) {
+      if (waitForBarrier) {
+        _render();
+      } else {
+        // We schedule the _renderLoop as a microtask to allow the
+        // scheduleRendering method to terminate, due to the fact that it is
+        // generally invoked from inside a HtmlElement.attached method
+        scheduleMicrotask(_renderLoop);
+      }
+    }
   }
 
   Future _render() async {
     await _barrier.next;
+    _renderLoop();
+  }
+
+  void _renderLoop() {
     while (_queue.isNotEmpty) {
       _queue.first.render();
       _queue.removeFirst();
     }
   }
 }