Run pub tests against older versions of barback.

third_party/pkg/barback-0.13.0/test/cancelable_future_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.cancelable_future_test;
 
 import 'dart:async';
 
+import 'package:barback/src/cancelable_future.dart';
 import 'package:barback/src/utils.dart';
-import 'package:barback/src/utils/cancelable_future.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 main() {
   initConfig();
 
   var completer;
   var future;
   setUp(() {
@@ skipping 32 matching lines @@
         completed = true;
       });
 
       future.cancel();
       completer.completeError("error");
 
       expect(pumpEventQueue().then((_) => completed), completion(isFalse));
     });
   });
 }