Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/transformer/lazy_many_to_one.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.lazy_many_to_one;
 
+import 'dart:async';
+
 import 'package:barback/barback.dart';
 
 import 'many_to_one.dart';
 
 /// Like [ManyToOneTransformer], but returns a lazy asset that doesn't perform
 /// the conglomeration until it's materialized.
 class LazyManyToOneTransformer extends ManyToOneTransformer
     implements LazyTransformer {
   LazyManyToOneTransformer(String extension)
       : super(extension);
 
-  void declareOutputs(DeclaringTransform transform) {
+  Future declareOutputs(DeclaringTransform transform) {
     transform.declareOutput(transform.primaryId.changeExtension(".out"));
   }
 }