Converted Observatory class-tree element

runtime/observatory/lib/src/elements/containers/virtual_tree.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:async';
 import 'dart:html';
+import 'dart:math' as Math;
 import 'package:observatory/src/elements/containers/virtual_collection.dart';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 import 'package:observatory/src/elements/helpers/tag.dart';
 
 typedef HtmlElement VirtualTreeCreateCallback(
     toggle({bool autoToggleSingleChildNodes, bool autoToggleWholeTree}));
 typedef void VirtualTreeUpdateCallback(HtmlElement el, dynamic item, int depth);
 typedef Iterable<dynamic> VritualTreeGetChildrenCallback(dynamic value);
 
+void virtualTreeUpdateLines(SpanElement element, int n) {
+  n = Math.max(0, n);
+  while (element.children.length > n) {
+    element.children.removeLast();
+  }
+  while (element.children.length < n) {
+    element.children.add(new SpanElement());
+  }
+}
+
 class VirtualTreeElement extends HtmlElement implements Renderable {
   static const tag =
       const Tag<VirtualTreeElement>('virtual-tree', dependencies: const [
         VirtualCollectionElement.tag
       ]);
 
   RenderingScheduler<VirtualTreeElement> _r;
 
   Stream<RenderedEvent<VirtualTreeElement>> get onRendered => _r.onRendered;
 
@@ skipping 114 matching lines @@
           depth++;
           return children.expand(_toDepth).toList()
               ..insert(0, --depth);
         }
       }
       return [depth];
     }
     _depths = _items.expand(_toDepth).toList();
   }
 }