Make IOSink implement StringSink

tests/standalone/io/process_broken_pipe_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.
 //
 // Process test program to test closed stdin from child process.
 
 import "dart:io";
 import "dart:isolate";
 
 import "process_test_util.dart";
 
 main() {
   // Running dart without arguments makes it close right away.
   var future = Process.start(new Options().executable, []);
   future.then((process) {
     process.stdin.done.catchError((e) {
       // Accept errors on stdin.
     });
 
     // Drain stdout and stderr.
     process.stdout.listen((_) {});
     process.stderr.listen((_) {});
 
     // Write to the stdin after the process is terminated to test
     // writing to a broken pipe.
     process.exitCode.then((code) {
-      process.stdin.add([0]);
+      process.stdin.writeBytes([0]);
     });
   });
 }