Make IOSink implement StringSink

tests/standalone/io/http_keep_alive_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 10 matching lines @@
       });
 }
 
 Future<HttpServer> startServer() {
   return HttpServer.bind("127.0.0.1", 0).then((server) {
     server.listen((request) {
       bool chunked = request.queryParameters["chunked"] == "true";
       int length = int.parse(request.queryParameters["length"]);
       var buffer = new List<int>.filled(length, 0);
       if (!chunked) request.response.contentLength = length;
-      request.response.add(buffer);
+      request.response.writeBytes(buffer);
       request.response.close();
     });
     return server;
   });
 }
 
 testKeepAliveNonChunked() {
   startServer().then((server) {
     var client = new HttpClient();
 
@@ skipping 45 matching lines @@
         });
 
   });
 }
 
 void main() {
   testKeepAliveNonChunked();
   testKeepAliveChunked();
   testKeepAliveMixed();
 }