Make IOSink implement StringSink

tests/standalone/io/file_system_links_test.dart

Index: tests/standalone/io/file_system_links_test.dart
diff --git a/tests/standalone/io/file_system_links_test.dart b/tests/standalone/io/file_system_links_test.dart
index 652bf78d21a26037649ac4ce383000b9b6ecaf73..bd5d252b5d8f37f31fb757d6c1c73c53c3e094c8 100644
--- a/tests/standalone/io/file_system_links_test.dart
+++ b/tests/standalone/io/file_system_links_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.
 
@@ -68,7 +68,7 @@ testFileWriteRead() {
   createLink(x, y, true, () {
     var data = "asdf".codeUnits;
     var output = new File(y).openWrite(FileMode.WRITE);
-    output.add(data);
+    output.writeBytes(data);
     output.close();
     output.done.then((_) {
       var read = new File(y).readAsBytesSync();