Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/standalone/io/process_test_util.dart

 // Copyright (c) 2012, 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.
 
 library process_test_util;
 
 import "dart:io";
 
 String getPlatformExecutableExtension() {
   var os = Platform.operatingSystem;
   if (os == 'windows') return '.exe';
   return '';  // Linux and Mac OS.
 }
 
 String getProcessTestFileName() {
   var extension = getPlatformExecutableExtension();
   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();
 }