Make IOSink implement StringSink

tests/standalone/io/http_head_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:io";
 
 void testHEAD(int totalConnections) {
   HttpServer.bind().then((server) {
     server.listen((request) {
         var response = request.response;
         if (request.uri.path == "/test100") {
           response.contentLength = 100;
           response.close();
         } else if (request.uri.path == "/test200") {
           response.contentLength = 200;
           List<int> data = new List<int>.filled(200, 0);
-          response.add(data);
+          response.writeBytes(data);
           response.close();
         } else if (request.uri.path == "/testChunked100") {
           List<int> data = new List<int>.filled(100, 0);
-          response.add(data);
+          response.writeBytes(data);
           response.close();
         } else if (request.uri.path == "/testChunked200") {
           List<int> data = new List<int>.filled(200, 0);
-          response.add(data);
+          response.writeBytes(data);
           response.close();
         } else {
           assert(false);
         }
       });
 
     HttpClient client = new HttpClient();
 
     int count = 0;
 
@@ skipping 25 matching lines @@
             (_) => Expect.fail("Data from HEAD request"),
             onDone: requestDone);
         });
     }
   });
 }
 
 void main() {
   testHEAD(4);
 }