Make IOSink implement StringSink

tests/standalone/io/https_client_certificate_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:uri";
 import "dart:isolate";
 
 const SERVER_ADDRESS = "127.0.0.1";
 const HOST_NAME = "localhost";
 
 
 Function test() {
   var keepAlive = new ReceivePort();
   HttpServer.bindSecure(SERVER_ADDRESS,
                         0,
                         backlog: 5,
                         certificateName: 'localhost_cert',
                         requestClientCertificate: true).then((server) {
     server.listen((HttpRequest request) {
       Expect.isNotNull(request.certificate);
       Expect.equals('CN=localhost', request.certificate.subject);
-      request.response.addString("Hello");
+      request.response.write("Hello");
       request.response.close();
     });
 
     HttpClient client = new HttpClient();
     client.getUrl(Uri.parse("https://$HOST_NAME:${server.port}/"))
         .then((request) => request.close())
         .then((response) {
           Expect.equals('CN=localhost', response.certificate.subject);
           Expect.equals('CN=myauthority', response.certificate.issuer);
           return response.reduce(<int>[],
@@ skipping 13 matching lines @@
   var testPkcertDatabase =
       new Path(new Options().script).directoryPath.append('pkcert/');
   SecureSocket.initialize(database: testPkcertDatabase.toNativePath(),
                           password: 'dartdart');
 }
 
 void main() {
   InitializeSSL();
   test();
 }