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

pkg/barback/lib/src/asset_node.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_node;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'errors.dart';
-import 'phase.dart';
 import 'transform_node.dart';
 
 /// Describes the current state of an asset as part of a transformation graph.
 ///
 /// An asset node can be in one of three states (see [AssetState]). It provides
 /// an [onStateChange] stream that emits an event whenever it changes state.
 ///
 /// Asset nodes are controlled using [AssetNodeController]s.
 class AssetNode {
   /// The id of the asset that this node represents.
@@ skipping 169 matching lines @@
 
   /// Whether this state is [AssetState.DIRTY].
   bool get isDirty => this == AssetState.DIRTY;
 
   final String name;
 
   const AssetState._(this.name);
 
   String toString() => name;
 }