Restore "Speedup Observatory elements first time rendering"

runtime/observatory/lib/src/elements/helpers/rendering_queue.dart

Index: runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
diff --git a/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart b/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
index 707de2331ad3883bd0fca81ef112073aaacacb20..e2446192a44baaab5d1eb034bbfeecbf26bcb10e 100644
--- a/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
+++ b/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
@@ -56,15 +56,29 @@ class RenderingQueue {
   /// 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);
-    // If no task are in the queue there is no rendering phase scheduled.
-    if (isEmpty) _render();
+    final wasEmpty = _queue.isEmpty;
     _queue.addLast(r);
+    // If no task are in the queue there is no rendering phase scheduled.
+    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();