Remove barback from the repo.

pkg/barback/test/transformer/conditionally_consume_primary.dart

-// Copyright (c) 2014, 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.conditionally_consume_primary;
-
-import 'dart:async';
-
-import 'package:barback/barback.dart';
-
-import 'rewrite.dart';
-
-/// A transformer that consumes its primary input only if its contents match a
-/// given pattern.
-class ConditionallyConsumePrimaryTransformer extends RewriteTransformer {
-  final Pattern content;
-
-  ConditionallyConsumePrimaryTransformer(String from, String to, this.content)
-      : super(from, to);
-
-  Future doApply(Transform transform) {
-    return getPrimary(transform).then((primary) {
-      return primary.readAsString().then((value) {
-        if (value.contains(content)) transform.consumePrimary();
-        return super.doApply(transform);
-      });
-    });
-  }
-}