Make IOSink implement StringSink

tests/standalone/io/regress_6521_test.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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.
 //
 // Regression test for http://code.google.com/p/dart/issues/detail?id=6393.
 
 import "dart:io";
 import "dart:uri";
 
 var client = new HttpClient();
 var clientRequest;
 
 void main() {
   HttpServer.bind("127.0.0.1", 0)
       .then((server) {
         server.listen(
             (req) {
               req.pipe(req.response);
             });
 
         client.openUrl("POST", Uri.parse("http://localhost:${server.port}/"))
             .then((request) {
               // Keep a reference to the client request object.
               clientRequest = request;
-              request.add([0]);
+              request.writeBytes([0]);
               return request.response;
             })
             .then((response) {
               // Wait with closing the client request until the response headers
               // are done.
               clientRequest.close();
               response.listen(
                   (_) {},
                   onDone: () {
                     client.close();
                     server.close();
                   });
             });
       });
 }