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

tests/standalone/io/secure_session_resume_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.
 //
 // This test tests TLS session resume, by making multiple client connections
 // on the same port to the same server, with a delay of 200 ms between them.
 // The unmodified secure_server_test creates all sessions simultaneously,
 // which means that no handshake completes and caches its keys in the session
 // cache in time for other connections to use it.
 //
@@ skipping 11 matching lines @@
 
 const SERVER_ADDRESS = "127.0.0.1";
 const HOST_NAME = "localhost";
 const CERTIFICATE = "localhost_cert";
 Future<SecureServerSocket> startServer() {
   return SecureServerSocket.bind(SERVER_ADDRESS,
                                  0,
                                  5,
                                  CERTIFICATE).then((server) {
     server.listen((SecureSocket client) {
-      client.reduce(<int>[], (message, data) => message..addAll(data))
+      client.fold(<int>[], (message, data) => message..addAll(data))
           .then((message) {
             String received = new String.fromCharCodes(message);
             Expect.isTrue(received.contains("Hello from client "));
             String name = received.substring(received.indexOf("client ") + 7);
             client.write("Welcome, client $name");
             client.close();
           });
     });
     return server;
   });
 }
 
 Future testClient(server, name) {
   return SecureSocket.connect(HOST_NAME, server.port).then((socket) {
     socket.write("Hello from client $name");
     socket.close();
-    return socket.reduce(<int>[], (message, data) => message..addAll(data))
+    return socket.fold(<int>[], (message, data) => message..addAll(data))
         .then((message) {
           Expect.listEquals("Welcome, client $name".codeUnits, message);
           return server;
         });
   });
 }
 
 void main() {
   Path scriptDir = new Path(new Options().script).directoryPath;
   Path certificateDatabase = scriptDir.append('pkcert');
   SecureSocket.initialize(database: certificateDatabase.toNativePath(),
                           password: 'dartdart');
 
   Duration delay = const Duration(milliseconds: 0);
   Duration delay_between_connections = const Duration(milliseconds: 300);
 
   startServer()
       .then((server) => Future.wait(
           ['able', 'baker', 'charlie', 'dozen', 'elapse']
           .map((name) {
             delay += delay_between_connections;
             return new Future.delayed(delay, () => server)
             .then((server) => testClient(server, name));
           })))
       .then((servers) => servers.first.close());
 }