Make IOSink implement StringSink

tests/standalone/io/process_stderr_test.dart

Index: tests/standalone/io/process_stderr_test.dart
diff --git a/tests/standalone/io/process_stderr_test.dart b/tests/standalone/io/process_stderr_test.dart
index b38d1a15d8d1e264ad75c8e11d84102203799a01..38e47f45fc8b1010b5b838eb8d54d652861c9b5a 100644
--- a/tests/standalone/io/process_stderr_test.dart
+++ b/tests/standalone/io/process_stderr_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.
 //
@@ -46,7 +46,7 @@ void test(Future<Process> future, int expectedExitCode) {
     }
 
     process.stdout.listen((_) {});
-    process.stdin.add(data);
+    process.stdin.writeBytes(data);
     process.stdin.close();
     process.stderr.listen(readData);
   });