Remove deprecated StringBuffer.add, addAll and addCharCode.

tests/lib/async/stream_transform_test.dart

 // Copyright (c) 2011, 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 stream_transform_test;
 
 import 'dart:async';
 import '../../../pkg/unittest/lib/unittest.dart';
 import 'event_helper.dart';
 
@@ skipping 30 matching lines @@
     actual.onDone(expectAsync0(() {
       Expect.listEquals(expected.events, actual.events);
     }));
     actual.pause();
     input.replay(c);
     actual.resume();
   });
 
   test("closing after done", () {
     var controller = new StreamController();
-    var buffer = new StringBuffer();
     controller.stream.map((e) => e).transform(new StreamTransformer(
         handleData: (element, sink) { sink.add(element); },
         handleDone: (sink) { sink.close(); })
     ).listen(expectAsync1((e) => expect(e, equals("foo"))));
 
     controller.add("foo");
     // Should not crash.
     controller.close();
   });
 }