Make IOSink implement StringSink

tests/standalone/io/regress_8828_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';
 
 void main() {
   HttpServer.bind().then((server) {
     server.listen((request) {
       request.response
-        ..addString("first line\n")
-        ..addString("")
-        ..addString("second line\n")
+        ..writeln("first line")
+        ..write("")
+        ..writeln("second line")
         ..close();
     });
 
     HttpClient client = new HttpClient();
     client.get("127.0.0.1", server.port, "/")
         .then((HttpClientRequest request) {
           return request.close();
         })
         .then((HttpClientResponse response) {
             List<int> body = new List();
           response.listen(body.addAll,
                           onDone: () {
                             Expect.equals("first line\nsecond line\n",
                                           new String.fromCharCodes(body));
                             server.close();
                           });
         });
   });
 }