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

sdk/lib/html/dart2js/html_dart2js.dart

Index: sdk/lib/html/dart2js/html_dart2js.dart
diff --git a/sdk/lib/html/dart2js/html_dart2js.dart b/sdk/lib/html/dart2js/html_dart2js.dart
index b9986875f1b2674b2c819290ee9220d43df2e81e..1dd5d3aa47b2b3e14c22344eae54cd08775e165c 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -29045,8 +29045,10 @@ abstract class CssClassSet implements Set<String> {
    *
    * This is the Dart equivalent of jQuery's
    * [addClass](http://api.jquery.com/addClass/).
+   *
+   * Returns `true` if [value] was added to the set, otherwise `false`.
    */
-  void add(String value);
+  bool add(String value);
 
   /**
    * Remove the class [value] from element, and return true on successful
@@ -29121,7 +29123,7 @@ class _MultiElementCssClassSet extends CssClassSetImpl {
    *   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));
   }