Make IOSink implement StringSink

tests/standalone/io/http_client_connect_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 14 matching lines @@
             (data) {},
             onDone: server.close);
       });
   });
 }
 
 void testGetDataRequest() {
   HttpServer.bind().then((server) {
     var data = "lalala".codeUnits;
     server.listen((request) {
-      request.response.add(data);
+      request.response.writeBytes(data);
       request.pipe(request.response);
     });
 
     var client = new HttpClient();
     client.get("127.0.0.1", server.port, "/")
       .then((request) => request.close())
       .then((response) {
         int count = 0;
         response.listen(
           (data) => count += data.length,
@@ skipping 31 matching lines @@
         .catchError((error) => port.close(),
                     test: (error) => error is HttpParserException);
   });
 }
 
 void testGetDataServerClose() {
   var completer = new Completer();
   HttpServer.bind().then((server) {
     server.listen((request) {
       request.response.contentLength = 100;
-      request.response.addString("data");
-      request.response.addString("more data");
+      request.response.write("data");
+      request.response.write("more data");
       completer.future.then((_) => server.close());
     });
 
     var port = new ReceivePort();
     var client = new HttpClient();
     client.get("127.0.0.1", server.port, "/")
       .then((request) => request.close())
       .then((response) {
         // Close the (incomplete) response, now we have seen the response object.
         completer.complete(null);
@@ skipping 26 matching lines @@
 
 
 void main() {
   testGetEmptyRequest();
   testGetDataRequest();
   testGetInvalidHost();
   testGetServerClose();
   testGetDataServerClose();
   testPostEmptyRequest();
 }