Make IOSink implement StringSink

tests/standalone/io/process_std_io_script2.dart

Index: tests/standalone/io/process_std_io_script2.dart
diff --git a/tests/standalone/io/process_std_io_script2.dart b/tests/standalone/io/process_std_io_script2.dart
index 5c0501fa4b5561d189e6c33cf5c6534874134b22..b6ff0262e4d8c5ce2c19b86095836714ee32cda9 100644
--- a/tests/standalone/io/process_std_io_script2.dart
+++ b/tests/standalone/io/process_std_io_script2.dart
@@ -1,4 +1,4 @@
-// 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.
 //
@@ -9,9 +9,11 @@ import "dart:io";
 
 writeData(data, encoding, stream) {
   if (stream == "stdout") {
-    stdout.addString(data, encoding);
+    stdout.encoding = encoding;
+    stdout.write(data);
   } else if (stream == "stderr") {
-    stderr.addString(data, encoding);
+    stderr.encoding = encoding;
+    stderr.write(data);
   }
 }