Make IOSink implement StringSink

tests/standalone/io/process_std_io_script2.dart

-// 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.
 //
 // Utility script to echo strings in various formats to stdout or
 // stderr.
 
 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);
   }
 }
 
 main() {
   var asciiString = 'abc';
   var latin1String = 'æøå';
   var utf8String = new String.fromCharCodes([955]);
   var options = new Options();
   if (options.arguments.length > 1) {
     var stream = options.arguments[1];
     if (options.arguments[0] == "ascii") {
       writeData(asciiString, Encoding.ASCII, stream);
     } else if (options.arguments[0] == "latin1") {
       writeData(latin1String, Encoding.ISO_8859_1, stream);
     } else if (options.arguments[0] == "utf8") {
       writeData(utf8String, Encoding.UTF_8, stream);
     }
   }
 }