Remove barback from the repo.

pkg/barback/test/transformer/check_content.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.check_content;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'mock.dart';
-
-/// A transformer that modifies assets that contains the given content.
-class CheckContentTransformer extends MockTransformer {
-  final Pattern content;
-  final String addition;
-
-  CheckContentTransformer(this.content, this.addition);
-
-  bool doIsPrimary(AssetId id) => true;
-
-  Future doApply(Transform transform) {
-    return getPrimary(transform).then((primary) {
-      return primary.readAsString().then((value) {
-        if (!value.contains(content)) return;
-
-        transform.addOutput(
-            new Asset.fromString(primary.id, "$value$addition"));
-      });
-    });
-  }
-}