Make AudioContext work in iOS 7.

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 1dd5d3aa47b2b3e14c22344eae54cd08775e165c..ec9e4c41606952cc9122272346187a92d483c126 100644
--- a/sdk/lib/html/dart2js/html_dart2js.dart
+++ b/sdk/lib/html/dart2js/html_dart2js.dart
@@ -29046,7 +29046,10 @@ 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`.
+   * 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.
    */
   bool add(String value);