Added helper function to automatically dirty a RenderingScheduler on attribute change

runtime/observatory/lib/src/elements/nav/menu_item.dart

Index: runtime/observatory/lib/src/elements/nav/menu_item.dart
diff --git a/runtime/observatory/lib/src/elements/nav/menu_item.dart b/runtime/observatory/lib/src/elements/nav/menu_item.dart
index fd89f7e6419a75c3eb0d7a460a3d64879e141b38..27d9722aa8ed0faea1fd09c3cf2acbfcb46d6e5a 100644
--- a/runtime/observatory/lib/src/elements/nav/menu_item.dart
+++ b/runtime/observatory/lib/src/elements/nav/menu_item.dart
@@ -57,22 +57,8 @@ class NavMenuItemElement extends HtmlElement implements Renderable {
   String _link;
   String get label => _label;
   String get link => _link;
-  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 label(String value) => _label = _r.checkAndReact(_label, value);
+  set link(String value) => _link = _r.checkAndReact(_link, value);
 
 
   factory NavMenuItemElement(String label, {String link,