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

tests/standalone/io/secure_socket_bad_certificate_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_write --short_socket_read
 // The --short_socket_write option does not work with external server
 // www.google.dk.  Add this to the test when we have secure server sockets.
@@ skipping 33 matching lines @@
     return acceptCertificate;
   };
 
   return SecureSocket.connect(host,
                               443,
                               onBadCertificate: badCertificateCallback)
       .then((socket) {
         Expect.isTrue(acceptCertificate);
         socket.write("GET / HTTP/1.0\r\nHost: $host\r\n\r\n");
         socket.close();
-        return socket.reduce(<int>[], (message, data)  => message..addAll(data))
+        return socket.fold(<int>[], (message, data)  => message..addAll(data))
             .then((message) {
               String received = new String.fromCharCodes(message);
               Expect.isTrue(received.contains('</body></html>'));
             });
       }).catchError((e) {
         Expect.isFalse(acceptCertificate);
       });
 }