Restore "Speedup Observatory elements first time rendering"

runtime/observatory/tests/observatory_ui/ports/element_test.dart

Index: runtime/observatory/tests/observatory_ui/ports/element_test.dart
diff --git a/runtime/observatory/tests/observatory_ui/ports/element_test.dart b/runtime/observatory/tests/observatory_ui/ports/element_test.dart
index 54af9d2335542e5daec98f230fde98d0d7f20566..a78a6d2f855fb10627f8f878e5a931cbf41a1ab8 100644
--- a/runtime/observatory/tests/observatory_ui/ports/element_test.dart
+++ b/runtime/observatory/tests/observatory_ui/ports/element_test.dart
@@ -39,6 +39,7 @@ main() {
     document.body.append(e);
     await e.onRendered.first;
     expect(e.children.length, isNonZero, reason: 'has elements');
+    await e.onRendered.first;
     expect(e.querySelectorAll('.port-number').length, equals(elements.length));
     e.remove();
     await e.onRendered.first;