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

pkg/barback/lib/src/asset_set.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_set;
 
-import 'dart:async';
 import 'dart:collection';
-import 'dart:io';
 
 import 'asset.dart';
 import 'asset_id.dart';
 
 /// A set of [Asset]s with distinct IDs.
 ///
 /// This uses the [AssetId] of each asset to determine uniqueness, so no two
 /// assets with the same ID can be in the set.
 class AssetSet extends IterableBase<Asset> {
   final _assets = new Map<AssetId, Asset>();
@@ skipping 44 matching lines @@
   }
 
   /// If the set contains an [Asset] with [id], removes and returns it.
   Asset removeId(AssetId id) => _assets.remove(id);
 
   /// Removes all assets from the set.
   void clear() {
     _assets.clear();
   }
 }