Make IOSink implement StringSink

tests/standalone/io/file_output_stream_test.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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.
 // Testing file input stream, VM-only, standalone test.
 
 import "dart:io";
 import "dart:isolate";
 
 void testOpenOutputStreamSync() {
   Directory tempDirectory = new Directory('').createTempSync();
 
   // Create a port for waiting on the final result of this test.
   ReceivePort done = new ReceivePort();
   done.receive((message, replyTo) {
     tempDirectory.deleteSync();
     done.close();
   });
 
   String fileName = "${tempDirectory.path}/test";
   File file = new File(fileName);
   file.createSync();
   IOSink x = file.openWrite();
   var data = [65, 66, 67];
-  x.add(data);
+  x.writeBytes(data);
   x.close();
   x.done.then((_) {
     Expect.listEquals(file.readAsBytesSync(), data);
     file.deleteSync();
     done.toSendPort().send("done");
   });
 }
 
 
 main() {
   testOpenOutputStreamSync();
 }