Make IOSink implement StringSink

tests/standalone/io/http_request_pipeling_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:io";
 import "dart:uri";
 
 void main() {
   final int REQUEST_COUNT = 100;
   int count = 0;
   HttpServer.bind().then((server) {
     server.listen((HttpRequest request) {
       count++;
-      request.response.addString(request.uri.path);
+      request.response.write(request.uri.path);
       request.response.close();
       if (request.uri.path == "/done") {
         request.response.done.then((_) {
             Expect.equals(REQUEST_COUNT + 1, count);
           server.close();
         });
       }
     });
     Socket.connect("127.0.0.1", server.port).then((s) {
       s.listen((data) { });
       for (int i = 0; i < REQUEST_COUNT; i++) {
-        s.addString("GET /$i HTTP/1.1\r\nX-Header-1: 111\r\n\r\n");
+        s.write("GET /$i HTTP/1.1\r\nX-Header-1: 111\r\n\r\n");
       }
-      s.addString("GET /done HTTP/1.1\r\nConnection: close\r\n\r\n");
+      s.write("GET /done HTTP/1.1\r\nConnection: close\r\n\r\n");
       s.close();
     });
   });
 }