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

tests/standalone/io/http_session_test.dart

 // Copyright (c) 2013, 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.
 
 import 'dart:async';
 import 'dart:io';
 
 const SESSION_ID = "DARTSESSID";
 
 String getSessionId(List<Cookie> cookies) {
-  var id = cookies.reduce(null, (last, cookie) {
+  var id = cookies.fold(null, (last, cookie) {
     if (last != null) return last;
     if (cookie.name.toUpperCase() == SESSION_ID) {
       Expect.isTrue(cookie.httpOnly);
       return cookie.value;
     }
     return null;
   });
   Expect.isNotNull(id);
   return id;
 }
 
 Future<String> connectGetSession(
     HttpClient client, int port, [String session]) {
   return client.get("127.0.0.1", port, "/")
     .then((request) {
       if (session != null) {
         request.cookies.add(new Cookie(SESSION_ID, session));
       }
       return request.close();
     })
     .then((response) {
-      return response.reduce(getSessionId(response.cookies), (v, _) => v);
+      return response.fold(getSessionId(response.cookies), (v, _) => v);
     });
 }
 
 void testSessions(int sessionCount) {
   var client = new HttpClient();
   HttpServer.bind().then((server) {
     var sessions = new Set();
     server.listen((request) {
       sessions.add(request.session.id);
       request.response.close();
@@ skipping 103 matching lines @@
         });
       });
   });
 }
 
 void main() {
   testSessions(1);
   testTimeout(5);
   testSessionsData();
 }