Restore "Speedup Observatory elements first time rendering"

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

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:html';
 import 'package:unittest/unittest.dart';
 import 'package:observatory/src/elements/objectstore_view.dart';
 import '../mocks.dart';
 
 main() {
@@ skipping 22 matching lines @@
         getter: expectAsync((i) async {
       expect(i, equals(isolate));
       return store;
     }, count: 1));
     final objects = new ObjectRepositoryMock();
     final e = new ObjectStoreViewElement(
         vm, isolate, events, notifs, stores, objects);
     document.body.append(e);
     await e.onRendered.first;
     expect(e.children.length, isNonZero, reason: 'has elements');
+    await e.onRendered.first;
     expect(e.querySelectorAll('.memberItem').length, equals(fields.length));
     e.remove();
     await e.onRendered.first;
     expect(e.children.length, isZero, reason: 'is empty');
   });
 }