pkg/barback: removed unused imports, added a missing import

pkg/barback/test/package_graph/transform_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.package_graph.transform_test;
 
-import 'dart:async';
-
-import 'package:barback/barback.dart';
 import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
 
 main() {
   initConfig();
   test("gets a transformed asset with a different path", () {
     initGraph(["app|foo.blub"], {"app": [
       [new RewriteTransformer("blub", "blab")]
@@ skipping 1033 matching lines @@
       buildShouldSucceed();
 
       modifyAsset("pkg2|a.inc", "b");
       updateSources(["pkg2|a.inc"]);
       expectAsset("pkg1|b", "spread out");
       expectNoAsset("pkg1|c.done");
       buildShouldSucceed();
     });
   });
 }