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

tests/standalone/io/http_compression_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:io';
@@ skipping 10 matching lines @@
       client.get("localhost", server.port, "/")
           .then((request) {
             request.headers.set(HttpHeaders.ACCEPT_ENCODING, "gzip,deflate");
             return request.close();
           })
           .then((response) {
             Expect.equals("gzip",
                           response.headers.value(HttpHeaders.CONTENT_ENCODING));
             response
                 .transform(new ZLibInflater())
-                .reduce([], (list, b) {
+                .fold([], (list, b) {
                   list.addAll(b);
                   return list;
                 }).then((list) {
                   Expect.listEquals(data, list);
                   server.close();
                   client.close();
                 });
           });
     });
   }
@@ skipping 42 matching lines @@
   test('deflate  ,  gzip', true);
   test('abc,deflate  ,  gzip,def,,,ghi  ,jkl', true);
   test('xgzip', false);
   test('gzipx;', false);
 }
 
 void main() {
   testServerCompress();
   testAcceptEncodingHeader();
 }