Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/standalone/io/directory_list_nonexistent_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.
 //
 // Directory listing test that tests listSync on a missing directory.
 //
 // TODO(7157): Merge this test into directory_test.dart testListNonExistent()
 // when it no longer crashes on Windows, when issue 7157 is resolved.
 
 import "dart:io";
@@ skipping 13 matching lines @@
 
 void testListTooLongName() {
   var keepAlive = new ReceivePort();
   new Directory("").createTemp().then((d) {
     var subDirName = 'subdir';
     var subDir = new Directory("${d.path}/$subDirName");
     subDir.create().then((ignore) {
       // Construct a long string of the form
       // 'tempdir/subdir/../subdir/../subdir'.
       var buffer = new StringBuffer();
-      buffer.add(subDir.path);
+      buffer.write(subDir.path);
       for (var i = 0; i < 1000; i++) {
-        buffer.add("/../${subDirName}");
+        buffer.write("/../${subDirName}");
       }
       var long = new Directory("${buffer.toString()}");
       Expect.throws(() => long.listSync(),
                     (e) => e is DirectoryIOException);
       Expect.throws(() => long.listSync(recursive: true),
                     (e) => e is DirectoryIOException);
       d.deleteSync(recursive: true);
       keepAlive.close();
     });
   });
 }
 
 void main() {
   testListNonExistent();
   testListTooLongName();
 }