Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/examples/simple_transformer/lib/transformer.dart

 // Copyright (c) 2012, 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.
 
 import 'package:barback/barback.dart';
 
 import 'dart:async';
 
 class InsertCopyright extends Transformer {
   String copyright = "Copyright (c) 2014, the Example project authors.\n";
 
   // A constructor named "asPlugin" is required. It can be empty, but
   // it must be present. It is how pub determines that you want this
   // class to be publicly available as a loadable transformer plugin.
   InsertCopyright.asPlugin();
 
   Future<bool> isPrimary(Asset input) {
     return new Future.value(input.id.extension == '.txt');
   }
 
   Future apply(Transform transform) {
     return transform.primaryInput.readAsString().then((content) {
       var id = transform.primaryInput.id;
       String newContent = copyright + content;
       transform.addOutput(new Asset.fromString(id, newContent));
     });
   }
 }