Make IOSink implement StringSink

tests/standalone/io/socket_invalid_arguments_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";
 import "dart:isolate";
 
 class NotAnInteger {
   operator==(other) => other == 1;
   operator<(other) => other > 1;
@@ skipping 21 matching lines @@
       socket.listen(
           (_) { },
           onError: (error) {
             Expect.fail("Error on stream");
           },
           onDone: () {
             Expect.equals(1, errors);
             socket.destroy();
             server.close();
           });
-      socket.add(buffer);
+      socket.writeBytes(buffer);
     });
   });
 }
 
 testServerSocketCreation(address, port, backlog) {
   var server;
   var port = new ReceivePort();
   try {
     ServerSocket.bind(address, port, backlog)
         .then((_) { Expect.fail("ServerSocket bound"); });
@@ skipping 11 matching lines @@
   testAdd(42);
   // TODO(8233): Throw ArgumentError from API implementation.
   // testAdd([-1]);
   // testAdd([2222222222222222222222222222222]);
   // testAdd([1, 2, 3, null]);
   // testAdd([new NotAnInteger()]);
   testServerSocketCreation(123, 123, 123);
   testServerSocketCreation("string", null, null);
   testServerSocketCreation("string", 123, null);
 }