Added helper function to automatically dirty a RenderingScheduler on attribute change

runtime/observatory/lib/src/elements/nav/menu.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/src/elements/helpers/tag.dart';
 import 'package:observatory/src/elements/helpers/rendering_scheduler.dart';
 
 class NavMenuElement extends HtmlElement implements Renderable {
@@ skipping 38 matching lines @@
   RenderingScheduler _r;
 
   Stream<RenderedEvent<NavMenuElement>> get onRendered => _r.onRendered;
 
   String _label;
   String _link;
   bool _last;
   String get label => _label;
   String get link => _link;
   bool get last => _last;
-  set label(String value) {
-    if (_label != value) {
-      _label = value;
-      _r.dirty();
-    } else {
-      _r.scheduleNotification();
-    }
-  }
-  set link(String value) {
-    if (_link != value) {
-      _link = value;
-      _r.dirty();
-    } else {
-      _r.scheduleNotification();
-    }
-  }
-  set last(bool value) {
-    if (_last != value) {
-      _last = value;
-      _r.dirty();
-    } else {
-      _r.scheduleNotification();
-    }
-  }
-
+  set label(String value) => _label = _r.checkAndReact(_label, value);
+  set link(String value) => _link = _r.checkAndReact(_link, value);
+  set last(bool value) => _last = _r.checkAndReact(_link, value);
 
   factory NavMenuElement(String label, {String link, bool last: false,
                              RenderingQueue queue}) {
     assert(label != null);
     assert(last != null);
     NavMenuElement e = document.createElement(tag.name);
     e._r = new RenderingScheduler(e, queue: queue);
     e._label = label;
     e._link = link;
     e._last = last;
@@ skipping 28 matching lines @@
     if (!last) {
       children.add(
         new LIElement()
           ..classes = ['nav-menu_spacer']
           ..innerHtml = '&gt;'
       );
     }
     shadowRoot.children = children;
   }
 }