Use Timer and not AnimationFrame during Observatory UI tests

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 a6062c38ebd64201b88518d6397e5c505f15d19b..7be443d7bd02e7b591059e3f6d8e1c18c3d8e4f6 100644
--- a/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
+++ b/runtime/observatory/lib/src/elements/helpers/rendering_queue.dart
@@ -37,6 +37,22 @@ class RenderingBarrierMock implements RenderingBarrier {
   }
 }
 
+/// MOCK synchronization system for timed barrier triggering.
+class TimedRenderingBarrier implements RenderingBarrier {
+  final StreamController<num> _stream = new StreamController<num>.broadcast();
+  num _ms = 0;
+
+  Future<num> get next => _stream.stream.first;
+
+  TimedRenderingBarrier({num milliseconds: 1}) {
+    assert(milliseconds != null);
+    assert(milliseconds > 0);
+    new Timer.periodic(new Duration(milliseconds: milliseconds), (Timer t) {
+      _stream.add(_ms += milliseconds);
+    });
+  }
+}
+
 /// RenderingTask queuing and synchronization system.
 class RenderingQueue {
   final RenderingBarrier _barrier;