Make IOSink implement StringSink

tests/standalone/io/regress_6521_test.dart

Index: tests/standalone/io/regress_6521_test.dart
diff --git a/tests/standalone/io/regress_6521_test.dart b/tests/standalone/io/regress_6521_test.dart
index 73eb67a649244786f137d28f4ffa0bc626beb478..aa8a9647a14b0b23c67acdf7752629ddcde4699d 100644
--- a/tests/standalone/io/regress_6521_test.dart
+++ b/tests/standalone/io/regress_6521_test.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.
 //
@@ -22,7 +22,7 @@ void main() {
             .then((request) {
               // Keep a reference to the client request object.
               clientRequest = request;
-              request.add([0]);
+              request.writeBytes([0]);
               return request.response;
             })
             .then((response) {