Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/too_many_open_files_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.too_many_open_files_test;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
@@ skipping 35 matching lines @@
   var futures = [];
   for (var i = 0; i < 1000; i++) {
     var asset = new Asset.fromPath(id, filePath);
     futures.add(assetHandler(asset));
   }
 
   expect(Future.wait(futures).whenComplete(() {
     new Directory(tempDir).delete(recursive: true);
   }), completes);
 }