Remove deprecated StringBuffer.add, addAll and addCharCode.

pkg/scheduled_test/lib/src/descriptor/directory.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.
 
 library descriptor.file;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'package:pathos/path.dart' as path;
@@ skipping 72 matching lines @@
     var buffer = new StringBuffer();
     buffer.writeln(description);
     for (var entry in contents.take(contents.length - 1)) {
       var entryString = prefixLines(entry.describe(), prefix: '|   ')
           .replaceFirst('|   ', '|-- ');
       buffer.writeln(entryString);
     }
 
     var lastEntryString = prefixLines(contents.last.describe(), prefix: '    ')
         .replaceFirst('    ', "'-- ");
-    buffer.add(lastEntryString);
+    buffer.write(lastEntryString);
     return buffer.toString();
   }
 }