Add Iterable.fold (and Stream.fold) which replace `reduce`.

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;
 
 abstract class CssClassSet implements Set<String> {
 
   String toString() {
     return readClasses().join(' ');
@@ skipping 44 matching lines @@
   bool any(bool f(String element)) => readClasses().any(f);
 
   bool get isEmpty => readClasses().isEmpty;
 
   int get length => readClasses().length;
 
   dynamic reduce(dynamic initialValue,
       dynamic combine(dynamic previousValue, String element)) {
     return readClasses().reduce(initialValue, combine);
   }
+
+  dynamic fold(dynamic initialValue,
+      dynamic combine(dynamic previousValue, String element)) {
+    return readClasses().fold(initialValue, combine);
+  }
   // interface Collection - END
 
   // interface Set - BEGIN
   /**
    * Determine if this element contains the class [value].
    *
    * This is the Dart equivalent of jQuery's
    * [hasClass](http://api.jquery.com/hasClass/).
    */
   bool contains(String value) => readClasses().contains(value);
@@ skipping 135 matching lines @@
    */
   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);
 }