Remove barback from the repo.

pkg/barback/test/transformer/declare_assets.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.declare_asset;
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// A transformer that declares some outputs and emits others.
-class DeclareAssetsTransformer extends MockTransformer
-    implements DeclaringTransformer {
-  /// The assets that the transformer declares that it will emit.
-  final List<AssetId> declared;
-
-  /// The assets that the transformer actually emits.
-  ///
-  /// These assets' contents will be identical to their ids.
-  final List<AssetId> emitted;
-
-  /// If this is non-`null`, assets are only declared for this input.
-  final AssetId input;
-
-  DeclareAssetsTransformer(Iterable<String> declared, {Iterable<String> emitted,
-        String input})
-      : this.declared = declared.map((id) => new AssetId.parse(id)).toList(),
-        this.emitted = (emitted == null ? declared : emitted)
-            .map((id) => new AssetId.parse(id)).toList(),
-        this.input = input == null ? null : new AssetId.parse(input);
-
-  bool doIsPrimary(AssetId id) => input == null || id == input;
-
-  void doApply(Transform transform) {
-    for (var id in emitted) {
-      transform.addOutput(new Asset.fromString(id, id.toString()));
-    }
-  }
-
-  void declareOutputs(DeclaringTransform transform) {
-    declared.forEach(transform.declareOutput);
-  }
-}