Make IOSink implement StringSink

tests/standalone/io/process_std_io_script.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.
 //
 // Utility script to echo stdin to stdout or stderr or both.
 
 import "dart:io";
 
 main() {
   var options = new Options();
   if (options.arguments.length > 0) {
     if (options.arguments[0] == "0") {
       stdin.pipe(stdout);
     } else if (options.arguments[0] == "1") {
       stdin.pipe(stderr);
     } else if (options.arguments[0] == "2") {
       stdin.listen((data) {
-        stdout.add(data);
-        stderr.add(data);
+        stdout.writeBytes(data);
+        stderr.writeBytes(data);
       });
     }
   }
 }