Make IOSink implement StringSink

tests/standalone/io/process_test_util.dart

Index: tests/standalone/io/process_test_util.dart
diff --git a/tests/standalone/io/process_test_util.dart b/tests/standalone/io/process_test_util.dart
index 6abf0c6849e6f71c6ccadc78b232816b33407982..f60c23948c352b23020db32d032f90cf0d66207f 100644
--- a/tests/standalone/io/process_test_util.dart
+++ b/tests/standalone/io/process_test_util.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.
 
@@ -17,6 +17,6 @@ String getProcessTestFileName() {
   var executable = new Options().executable;
   var dirIndex = executable.lastIndexOf('dart$extension');
   var buffer = new StringBuffer(executable.substring(0, dirIndex));
-  buffer.add('process_test$extension');
+  buffer.write('process_test$extension');
   return buffer.toString();
 }