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

pkg/barback/test/package_graph/source_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("gets a source asset", () {
     initGraph(["app|foo.txt"]);
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt");
@@ skipping 123 matching lines @@
     // Now update an asset that does trigger a transformer.
     updateSources(["app|foo.txt"]);
 
     resumeProvider();
 
     buildShouldSucceed();
 
     expect(transformer.numRuns, completion(equals(2)));
   });
 }