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

pkg/barback/lib/src/package_graph.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.package_graph;
 
 import 'dart:async';
 
-import 'package:stack_trace/stack_trace.dart';
-
-import 'asset.dart';
 import 'asset_cascade.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'build_result.dart';
 import 'errors.dart';
 import 'package_provider.dart';
 import 'utils.dart';
 
 /// The collection of [AssetCascade]s for an entire application.
 ///
@@ skipping 90 matching lines @@
   /// Removes [removed] from the graph's known set of source assets.
   void removeSources(Iterable<AssetId> sources) {
     groupBy(sources, (id) => id.package).forEach((package, ids) {
       var cascade = _cascades[package];
       if (cascade == null) throw new ArgumentError("Unknown package $package.");
       _cascadeResults[package] = null;
       cascade.removeSources(ids);
     });
   }
 }