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..b54623ee20e9f993bf9c19511de1d5055db6dc45 100644
--- a/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
+++ b/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
@@ -56,10 +56,17 @@ 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 realtime: false}) {

[siva, 2017/08/14 20:27:18]

instead of calling enqueue here with a parameter realtime why not just call
scheduleMicrotask(r.render) in the caller.

[cbernaschina, 2017/08/14 21:08:49]

See new Patch.

The execution on a browser which supports custom elements is as follow:
 - the root component schedules the first task and initializes the rendering
stage
 - all the children schedule an extra task that is executed just after the
parent one
   (due to the fact that the render methods are synchronous all the components
will render during a single Microtask)

The execution on a browser with partial support of the custom elements is as
follow:
 - the root component schedules the first task and initializes the rendering
stage
 - all the children schedule an extra task that is executed via Microtask,
possibly packing groups of them together.

Forwarding the decision to the enqueue method guarantees that the extra
Microtasks are created just as a fallback and not as part of the normal
behavior.

     assert(r != null);
     // If no task are in the queue there is no rendering phase scheduled.
-    if (isEmpty) _render();
+    if (isEmpty) {
+      if (realtime) {
+        scheduleMicrotask(r.render);
+        return;
+      } else {
+        _render();
+      }
+    }
     _queue.addLast(r);
   }