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

tools/dom/src/CssClassSet.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;
 
 /** A Set that stores the CSS class names for an element. */
 abstract class CssClassSet implements Set<String> {
 
   /**
@@ skipping 17 matching lines @@
    * This is the Dart equivalent of jQuery's
    * [hasClass](http://api.jquery.com/hasClass/).
    */
   bool contains(String value);
 
   /**
    * Add the class [value] to element.
    *
    * This is the Dart equivalent of jQuery's
    * [addClass](http://api.jquery.com/addClass/).
+   *
+   * If this corresponds to one element. Returns `true` if [value] was added to
+   * the set, otherwise `false`.
+   *
+   * If this corresponds to many elements, `null` is always returned.
    */
-  void add(String value);
+  bool add(String 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 54 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)) {
     _elementCssClassSetIterable.forEach((e) => e.modify(f));
   }
 
   /**
    * Adds the class [value] to the element if it is not on it, removes it if it
    * is.
    */
   bool toggle(String value, [bool shouldAdd]) =>
       _modifyWithReturnValue((e) => e.toggle(value, shouldAdd));
 
@@ skipping 27 matching lines @@
       }
     }
     return s;
   }
 
   void writeClasses(Set<String> s) {
     List list = new List.from(s);
     _element.className = s.join(' ');
   }
 }