Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/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);
       });
     });
   }
 }