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

pkg/barback/lib/src/asset_id.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.asset_id;
 
-import 'dart:async';
-import 'dart:io';
-
 import 'package:path/path.dart' as pathos;
 
 /// AssetIDs always use POSIX style paths regardless of the host platform.
 final _posix = new pathos.Builder(style: pathos.Style.posix);
 
 /// Identifies an asset within a package.
 class AssetId implements Comparable<AssetId> {
   /// The name of the package containing this asset.
   final String package;
 
@@ skipping 68 matching lines @@
   /// but with file extension [newExtension].
   AssetId changeExtension(String newExtension) =>
       new AssetId(package, pathos.withoutExtension(path) + newExtension);
 
   String toString() => "$package|$path";
 
   /// Serializes this [AssetId] to an object that can be sent across isolates
   /// and passed to [deserialize].
   serialize() => [package, path];
 }