Remove barback from the repo.

pkg/barback/test/transformer/bad.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 barback.test.transformer.bad;
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// A transformer that throws an exception when run, after generating the
-/// given outputs.
-class BadTransformer extends MockTransformer {
-  /// The error it throws.
-  static const ERROR = "I am a bad transformer!";
-
-  /// The list of asset names that it should output.
-  final List<String> outputs;
-
-  BadTransformer(this.outputs);
-
-  bool doIsPrimary(AssetId id) => true;
-
-  void doApply(Transform transform) {
-    // Create the outputs first.
-    for (var output in outputs) {
-      var id = new AssetId.parse(output);
-      transform.addOutput(new Asset.fromString(id, output));
-    }
-
-    // Then fail.
-    throw ERROR;
-  }
-}