Remove barback from the repo.

pkg/barback/test/transformer/declaring_check_content_and_rename.dart

-// Copyright (c) 2014, 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 barback.test.transformer.declaring_check_content_and_rename;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'check_content_and_rename.dart';
-
-class DeclaringCheckContentAndRenameTransformer
-    extends CheckContentAndRenameTransformer
-    implements DeclaringTransformer {
-  DeclaringCheckContentAndRenameTransformer({String oldExtension,
-        String oldContent, String newExtension, String newContent})
-      : super(oldExtension: oldExtension, oldContent: oldContent,
-              newExtension: newExtension, newContent: newContent);
-
-  void declareOutputs(DeclaringTransform transform) {
-    transform.declareOutput(
-        transform.primaryId.changeExtension('.$newExtension'));
-  }
-}