Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/package_graph/repetition_test.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.package_graph.transform_test;
 
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
@@ skipping 56 matching lines @@
     for (var i = 0; i < 1000; i++) {
       updateTransformers("app", [[rewrite]]);
       updateTransformers("app", [[]]);
     }
 
     expectAsset("app|foo.txt", "foo");
     expectNoAsset("app|foo.out");
     buildShouldSucceed();
   });
 }