Make IOSink implement StringSink

tests/standalone/io/process_stdin_transform_unsubscribe_test.dart

 // 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.
 //
 // Process test program to test process communication.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:math';
 
 import "process_test_util.dart";
 
 void test(Future<Process> future, int expectedExitCode) {
   future.then((process) {
     process.exitCode.then((exitCode) {
       Expect.equals(expectedExitCode, exitCode);
     });
 
     process.stdout.listen((_) {});
     process.stderr.listen((_) {});
-    process.stdin.addString("Line1\n");
+    process.stdin.writeln("Line1");
   });
 }
 
 main() {
   var scriptName = "process_stdin_transform_unsubscribe_script.dart";
   var scriptFile = new File("tests/standalone/io/$scriptName");
   if (!scriptFile.existsSync()) {
     scriptFile = new File("../tests/standalone/io/$scriptName");
   }
   Expect.isTrue(scriptFile.existsSync());
   test(Process.start(new Options().executable, [scriptFile.path]), 0);
 }