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

pkg/barback/test/asset_set_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_set_test;
 
-import 'dart:async';
-import 'dart:io';
-
 import 'package:barback/barback.dart';
 import 'package:barback/src/asset_set.dart';
 import 'package:unittest/unittest.dart';
 
 import 'utils.dart';
 
 main() {
   initConfig();
 
   var fooId = new AssetId.parse("app|foo.txt");
@@ skipping 124 matching lines @@
 
     test("returns null when removing an asset not in the set", () {
       var set = new AssetSet();
       var foo = new Asset.fromString(fooId, "foo");
       set.add(foo);
 
       expect(set.removeId(barId), isNull);
     });
   });
 }