Make IOSink implement StringSink

tests/standalone/io/secure_socket_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";
 import "dart:io";
 
 Future<HttpServer> startServer() {
   return HttpServer.bindSecure(
       "127.0.0.1",
       0,
       backlog: 5,
       certificateName: 'localhost_cert').then((server) {
     server.listen((HttpRequest request) {
       request.listen(
         (_) { },
         onDone: () {
           request.response.contentLength = 100;
           for (int i = 0; i < 10; i++) {
-            request.response.add([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+            request.response.writeBytes([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
           }
           request.response.close();
         });
     });
     return server;
   });
 }
 
 void InitializeSSL() {
   var testPkcertDatabase =
       new Path(new Options().script).directoryPath.append('pkcert/');
   SecureSocket.initialize(database: testPkcertDatabase.toNativePath(),
                           password: 'dartdart');
 }
 
 void main() {
   InitializeSSL();
   List<int> body = <int>[];
   startServer().then((server) {
     SecureSocket.connect("localhost", server.port).then((socket) {
-      socket.add("GET / HTTP/1.0\r\nHost: localhost\r\n\r\n".codeUnits);
+      socket.write("GET / HTTP/1.0\r\nHost: localhost\r\n\r\n");
       socket.close();
       socket.listen(
         (List<int> data) {
           body.addAll(data);
         },
         onDone: () {
           Expect.isTrue(body.length > 100, "$body\n${body.length}");
           Expect.equals(72, body[0]);
           Expect.equals(9, body[body.length - 1]);
           server.close();
         },
         onError: (e) => Expect.fail("Unexpected error $e"));
     });
   });
 }