Restore "Speedup Observatory elements first time rendering"

runtime/observatory/tests/observatory_ui/inbound_references/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/curly_block.dart';
 import 'package:observatory/src/elements/inbound_references.dart';
 import 'package:observatory/src/elements/instance_ref.dart';
 import '../mocks.dart';
@@ skipping 25 matching lines @@
       expect(i, equals(isolate));
       expect(id, equals(object.id));
       invoked = true;
       return references;
     }, count: 1));
     final objects = new ObjectRepositoryMock();
     final e = new InboundReferencesElement(isolate, object, inbounds, objects);
     document.body.append(e);
     await e.onRendered.first;
     expect(invoked, isFalse);
-    expect(e.children.length, isNonZero, reason: 'has elements');
+    expect(e.children, isNotEmpty, reason: 'has elements');
     expect(e.querySelectorAll(iTag).length, isZero);
     (e.querySelector(cTag) as CurlyBlockElement).toggle();
     await e.onRendered.first;
     expect(invoked, isTrue);
     expect(e.querySelectorAll(iTag).length, equals(1));
     expect(e.querySelectorAll(rTag).length, equals(1));
     e.remove();
     await e.onRendered.first;
-    expect(e.children.length, isZero, reason: 'is empty');
+    expect(e.children, isEmpty, reason: 'is empty');
   });
 }