Speedup Observatory elements first time rendering

R=asiva@google.com, bkonyi@google.com

Review-Url: https://codereview.chromium.org/2995883002 .

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();

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

@@ -32,6 +32,7 @@ class RenderingScheduler<T extends Renderable> implements RenderingTask {
   bool _dirty = false;
   bool _renderingScheduled = false;
   bool _notificationScheduled = false;
+  bool _waitForBarrier = false;
 
   /// Element managed by this scheduler.
   final T element;
@@ -103,7 +104,8 @@ class RenderingScheduler<T extends Renderable> implements RenderingTask {
   void scheduleRendering() {
     if (_renderingScheduled) return;
     if (!_enabled) return;
-    queue.enqueue(this);
+    queue.enqueue(this, waitForBarrier: _waitForBarrier);
+    _waitForBarrier = true;
     _renderingScheduled = true;
   }