Remove barback from the repo.

pkg/barback/test/transformer/has_input.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.has_input;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// Overwrites its primary inputs with descriptions of whether various secondary
-/// inputs exist.
-class HasInputTransformer extends MockTransformer {
-  /// The inputs whose existence will be checked.
-  final List<AssetId> inputs;
-
-  HasInputTransformer(Iterable<String> inputs)
-      : inputs = inputs.map((input) => new AssetId.parse(input)).toList();
-
-  bool doIsPrimary(AssetId id) => true;
-
-  Future doApply(Transform transform) {
-    return Future.wait(inputs.map((input) {
-      return transform.hasInput(input).then((hasInput) => "$input: $hasInput");
-    })).then((results) {
-      transform.addOutput(new Asset.fromString(
-          transform.primaryInput.id, results.join(', ')));
-    });
-  }
-
-  String toString() => "has inputs $inputs";
-}