Remove barback from the repo.

pkg/barback/test/transformer/catch_asset_not_found.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.catch_asset_not_found;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// A transformer that tries to load a secondary input and catches an
-/// [AssetNotFoundException] if the input doesn't exist.
-class CatchAssetNotFoundTransformer extends MockTransformer {
-  /// The extension of assets this applies to.
-  final String extension;
-
-  /// The id of the secondary input to load.
-  final AssetId input;
-
-  CatchAssetNotFoundTransformer(this.extension, String input)
-      : input = new AssetId.parse(input);
-
-  bool doIsPrimary(AssetId id) => id.extension == extension;
-
-  Future doApply(Transform transform) {
-    return transform.getInput(input).then((_) {
-      transform.addOutput(new Asset.fromString(
-          transform.primaryInput.id, "success"));
-    }).catchError((e) {
-      if (e is! AssetNotFoundException) throw e;
-      transform.addOutput(new Asset.fromString(
-          transform.primaryInput.id, "failed to load $input"));
-    });
-  }
-}