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

tests/standalone/io/http_keep_alive_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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "dart:async";
 import "dart:io";
 
 Future getData(HttpClient client, int port, bool chunked, int length) {
   return client.get("localhost", port, "/?chunked=$chunked&length=$length")
       .then((request) => request.close())
       .then((response) {
-        return response.reduce(0, (bytes, data) => bytes + data.length)
+        return response.fold(0, (bytes, data) => bytes + data.length)
             .then((bytes) {
               Expect.equals(length, bytes);
             });
       });
 }
 
 Future<HttpServer> startServer() {
   return HttpServer.bind("127.0.0.1", 0).then((server) {
     server.listen((request) {
       bool chunked = request.queryParameters["chunked"] == "true";
@@ skipping 59 matching lines @@
         });
 
   });
 }
 
 void main() {
   testKeepAliveNonChunked();
   testKeepAliveChunked();
   testKeepAliveMixed();
 }