Make IOSink implement StringSink

tests/standalone/io/process_set_exit_code_script.dart

Index: tests/standalone/io/process_set_exit_code_script.dart
diff --git a/tests/standalone/io/process_set_exit_code_script.dart b/tests/standalone/io/process_set_exit_code_script.dart
index f24796317fdc952b2c1eba3f5f8b7edaeca31348..9a34f052a14619eff51b43f7b0e620f0ee8dbf41 100644
--- a/tests/standalone/io/process_set_exit_code_script.dart
+++ b/tests/standalone/io/process_set_exit_code_script.dart
@@ -1,11 +1,11 @@
-// 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.
 
 import "dart:io";
 
 main() {
-  stdout.addString("standard out");
-  stderr.addString("standard error");
+  stdout.write("standard out");
+  stderr.write("standard error");
   exitCode = 25;
 }