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

pkg/barback/test/asset_id_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.asset_id_test;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 main() {
   initConfig();
   group("parse", () {
     test("parses the package and path", () {
       var id = new AssetId.parse("package|path/to/asset.txt");
@@ skipping 18 matching lines @@
     expect(new AssetId.parse("foo|asset.txt"), equals(
            new AssetId.parse("foo|asset.txt")));
 
     expect(new AssetId.parse("foo|asset.txt"), isNot(equals(
         new AssetId.parse("bar|asset.txt"))));
 
     expect(new AssetId.parse("foo|asset.txt"), isNot(equals(
         new AssetId.parse("bar|other.txt"))));
   });
 }