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

tests/standalone/io/secure_client_raw_server_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";
@@ skipping 26 matching lines @@
             bytesWritten += client.write(
                 dataToWrite, bytesWritten, dataToWrite.length - bytesWritten);
             if (bytesWritten < dataToWrite.length) {
               client.writeEventsEnabled = true;
             }
             if (bytesWritten == dataToWrite.length) {
               client.shutdown(SocketDirection.SEND);
             }
             break;
           case RawSocketEvent.READ_CLOSED:
-            dataToWrite = readChunks.reduce(<int>[], (list, x) {
+            dataToWrite = readChunks.fold(<int>[], (list, x) {
               list.addAll(x);
               return list;
             });
             client.writeEventsEnabled = true;
             break;
         }
       });
     });
     return server;
   });
@@ skipping 24 matching lines @@
   Path certificateDatabase = scriptDir.append('pkcert');
   SecureSocket.initialize(database: certificateDatabase.toNativePath(),
                           password: 'dartdart');
 
   startEchoServer()
       .then(testClient)
       .then((server) {
         server.close();
       });
 }