Restore "Speedup Observatory elements first time rendering"

runtime/observatory/lib/src/elements/retaining_path.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 'dart:async';
 import 'package:observatory/models.dart' as M;
 import 'package:observatory/src/elements/curly_block.dart';
 import 'package:observatory/src/elements/instance_ref.dart';
 import 'package:observatory/src/elements/helpers/any_ref.dart';
@@ skipping 43 matching lines @@
   }
 
   @override
   void detached() {
     super.detached();
     children = [];
     _r.disable(notify: true);
   }
 
   void render() {
-    children = [
-      new CurlyBlockElement(expanded: _expanded, queue: _r.queue)
-        ..content = _createContent()
-        ..onToggle.listen((e) async {
-          _expanded = e.control.expanded;
-          if (_expanded) {
-            e.control.disabled = true;
-            await _refresh();
-            e.control.disabled = false;
-          }
-        })
-    ];
+    final curlyBlock =
+        new CurlyBlockElement(expanded: _expanded, queue: _r.queue)
+          ..content = _createContent()
+          ..onToggle.listen((e) async {
+            _expanded = e.control.expanded;
+            if (_expanded) {
+              e.control.disabled = true;
+              await _refresh();
+              e.control.disabled = false;
+            }
+          });
+    children = [curlyBlock];
+    _r.waitFor([curlyBlock.onRendered.first]);
   }
 
   Future _refresh() async {
     _path = null;
     _path = await _retainingPaths.get(_isolate, _object.id);
     _r.dirty();
   }
 
   List<Element> _createContent() {
     if (_path == null) {
@@ skipping 38 matching lines @@
       ..classes = ['indent']
       ..children = content;
   }
 
   Element _createGCRootItem() {
     return new DivElement()
       ..classes = ['indent']
       ..text = 'retained by a GC root';
   }
 }