Fix interactive_flex demo

sky/sdk/lib/rendering/object.dart

 // Copyright 2015 The Chromium Authors. 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:math' as math;
 import 'dart:sky' as sky;
 import 'dart:sky' show Point, Offset, Size, Rect, Color, Paint, Path;
 
 import '../base/hit_test.dart';
 import '../base/node.dart';
@@ skipping 104 matching lines @@
     }
     _needsLayout = true;
     assert(_relayoutSubtreeRoot != null);
     if (_relayoutSubtreeRoot != this) {
       final parent = this.parent; // TODO(ianh): Remove this once the analyzer is cleverer
       assert(parent is RenderObject);
       parent.markNeedsLayout();
       assert(parent == this.parent); // TODO(ianh): Remove this once the analyzer is cleverer
     } else {
       _nodesNeedingLayout.add(this);
+      scheduler.ensureVisualUpdate();
     }
   }
   void _cleanRelayoutSubtreeRoot() {
     if (_relayoutSubtreeRoot != this) {
       _relayoutSubtreeRoot = null;
       _needsLayout = true;
       _cleanRelayoutSubtreeRootChildren();
     }
   }
   void _cleanRelayoutSubtreeRootChildren() { } // workaround for lack of inter-class mixins in Dart
@@ skipping 394 matching lines @@
     int count = 1;
     ChildType child = _firstChild;
     while (child != null) {
       result += '${prefix}child ${count}: ${child.toString(prefix)}';
       count += 1;
       child = child.parentData.nextSibling;
     }
     return result;
   }
 }