Remove barback from the repo.

pkg/barback/lib/src/internal_asset.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.
-
-/// This library exists so that pub can retain backwards-compatibility with
-/// versions of barback prior to 0.13.1.
-///
-/// In 0.13.1, `lib/src/internal_asset.dart` was moved to
-/// `lib/src/asset/internal_asset.dart. Pub needs to support all versions of
-/// barback back through 0.13.0, though. In order for this to work, it needs to
-/// be able to import "package:barback/src/internal_asset.dart" on all available
-/// barback versions, hence the existence of this library.
-library barback.internal_asset;
-
-export 'asset/internal_asset.dart';