Make IOSink implement StringSink

tests/standalone/io/secure_multiple_client_server_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";
 import "dart:isolate";
 
 const SERVER_ADDRESS = "127.0.0.1";
 const HOST_NAME = "localhost";
 const CERTIFICATE = "localhost_cert";
 Future<SecureServerSocket> startServer() {
   return SecureServerSocket.bind(SERVER_ADDRESS,
                                  0,
                                  5,
                                  CERTIFICATE).then((server) {
     server.listen((SecureSocket client) {
       client.reduce(<int>[], (message, data) => message..addAll(data))
           .then((message) {
             String received = new String.fromCharCodes(message);
             Expect.isTrue(received.contains("Hello from client "));
             String name = received.substring(received.indexOf("client ") + 7);
-            client.add("Welcome, client $name".codeUnits);
+            client.writeBytes("Welcome, client $name".codeUnits);
             client.close();
           });
     });
     return server;
   });
 }
 
 Future testClient(server, name) {
   return SecureSocket.connect(HOST_NAME, server.port).then((socket) {
-    socket.add("Hello from client $name".codeUnits);
+    socket.writeBytes("Hello from client $name".codeUnits);
     socket.close();
     return socket.reduce(<int>[], (message, data) => message..addAll(data))
         .then((message) {
           Expect.listEquals("Welcome, client $name".codeUnits, message);
           return server;
         });
   });
 }
 
 void main() {
   Path scriptDir = new Path(new Options().script).directoryPath;
   Path certificateDatabase = scriptDir.append('pkcert');
   SecureSocket.initialize(database: certificateDatabase.toNativePath(),
                           password: 'dartdart');
 
   startServer()
       .then((server) => Future.wait(
           ['able', 'baker', 'charlie', 'dozen', 'elapse']
           .map((name) => testClient(server, name))))
       .then((servers) => servers.first.close());
 }