Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/barback_mode_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.barback_mode_test;
 
 import 'package:barback/barback.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 main() {
   initConfig();
   test("constructor uses canonical instances for DEBUG and RELEASE", () {
     expect(identical(BarbackMode.DEBUG,
         new BarbackMode(BarbackMode.DEBUG.name)), isTrue);
     expect(identical(BarbackMode.RELEASE,
         new BarbackMode(BarbackMode.RELEASE.name)), isTrue);
   });
 }