Restore "Speedup Observatory elements first time rendering"

runtime/observatory/tests/observatory_ui/curly_block/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';
 
 main() {
   CurlyBlockElement.tag.ensureRegistration();
@@ skipping 49 matching lines @@
     });
     test('disabled', () {
       final e = new CurlyBlockElement(disabled: true);
       expect(e, isNotNull, reason: 'element correctly created');
       expect(e.expanded, isFalse);
       expect(e.disabled, isTrue);
     });
   });
   test('elements created', () async {
     final e = new CurlyBlockElement();
-    expect(e.children.length, isZero, reason: 'is empty');
+    expect(e.children, isEmpty, reason: 'is empty');
     document.body.append(e);
     await e.onRendered.first;
-    expect(e.children.length, isNonZero, reason: 'has elements');
+    expect(e.children, isNotEmpty, reason: 'has elements');
     e.remove();
     await e.onRendered.first;
-    expect(e.children.length, isZero, reason: 'is empty');
+    expect(e.children, isEmpty, reason: 'is empty');
   });
   group('content', () {
     CurlyBlockElement e;
     setUp(() async {
       e = new CurlyBlockElement();
       e.content = [document.createElement('content')];
       document.body.append(e);
       await e.onRendered.first;
     });
     tearDown(() {
@@ skipping 80 matching lines @@
       e.expanded = !e.expanded;
       await e.onRendered.first;
     });
     test('does not fire if setting same expanded value', () async {
       e.onToggle.listen(expectAsync((_) {}, count: 0));
       e.expanded = e.expanded;
       await e.onRendered.first;
     });
   });
 }