Restore "Speedup Observatory elements first time rendering"

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

Index: runtime/observatory/tests/observatory_ui/curly_block/element_test.dart
diff --git a/runtime/observatory/tests/observatory_ui/curly_block/element_test.dart b/runtime/observatory/tests/observatory_ui/curly_block/element_test.dart
index 068f7416cee17ca0923d31efe4d2e3f2e9fa8d1b..69e4063e37ff61b0dab37a4a171065befeb95347 100644
--- a/runtime/observatory/tests/observatory_ui/curly_block/element_test.dart
+++ b/runtime/observatory/tests/observatory_ui/curly_block/element_test.dart
@@ -67,13 +67,13 @@ main() {
   });
   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;