Make IOSink implement StringSink

tests/standalone/io/http_server_early_client_close_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.
 
 import "dart:async";
 import "dart:io";
 import "dart:isolate";
 
 void sendData(List<int> data, int port) {
   Socket.connect("127.0.0.1", port).then((socket) {
     socket.listen((data) {
         Expect.fail("No data response was expected");
       });
-    socket.add(data);
+    socket.writeBytes(data);
     socket.close();
     socket.done.then((_) {
       socket.destroy();
     });
   });
 }
 
 class EarlyCloseTest {
   EarlyCloseTest(this.data,
                  String this.exception,
@@ skipping 105 matching lines @@
       });
     }
     makeRequest();
   });
 }
 
 void main() {
   testEarlyClose1();
 //  testEarlyClose2();
 }