Set.add returns true if item has been added, otherwise false

sdk/lib/html/html_common/css_class_set.dart

 // Copyright (c) 2012, 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.
 
 part of html_common;
 
 abstract class CssClassSetImpl implements CssClassSet {
 
   String toString() {
     return readClasses().join(' ');
@@ skipping 74 matching lines @@
 
   /** Lookup from the Set interface. Not interesting for a String set. */
   String lookup(String value) => contains(value) ? value : null;
 
   /**
    * Add the class [value] to element.
    *
    * This is the Dart equivalent of jQuery's
    * [addClass](http://api.jquery.com/addClass/).
    */
-  void add(String value) {
+  bool add(String value) {
     // TODO - figure out if we need to do any validation here
     // or if the browser natively does enough.
-    modify((s) => s.add(value));
+    return modify((s) => s.add(value));
   }
 
   /**
    * Remove the class [value] from element, and return true on successful
    * removal.
    *
    * This is the Dart equivalent of jQuery's
    * [removeClass](http://api.jquery.com/removeClass/).
    */
   bool remove(Object value) {
@@ skipping 90 matching lines @@
 
   /**
    * Helper method used to modify the set of css classes on this element.
    *
    *   f - callback with:
    *   s - a Set of all the css class name currently on this element.
    *
    *   After f returns, the modified set is written to the
    *       className property of this element.
    */
-  void modify( f(Set<String> s)) {
+  modify( f(Set<String> s)) {
     Set<String> s = readClasses();
-    f(s);
+    var ret = f(s);
     writeClasses(s);
+    return ret;
   }
 
   /**
    * Read the class names from the Element class property,
    * and put them into a set (duplicates are discarded).
    * This is intended to be overridden by specific implementations.
    */
   Set<String> readClasses();
 
   /**
    * Join all the elements of a set into one string and write
    * back to the element.
    * This is intended to be overridden by specific implementations.
    */
   void writeClasses(Set<String> s);
 }