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

pkg/barback/test/package_graph/errors_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.source_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("errors if two transformers output the same file", () {
     initGraph(["app|foo.a"], {"app": [
@@ skipping 307 matching lines @@
 
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt", "foo");
     buildShouldSucceed();
 
     updateSources(["app|foo.in"]);
     expectAsset("app|foo.txt", "foo");
     buildShouldFail([isAssetCollisionException("app|foo.txt")]);
   });
 }