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

pkg/barback/lib/src/transform.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.transform;
 
 import 'dart:async';
 
 import 'asset.dart';
 import 'asset_id.dart';
 import 'asset_node.dart';
 import 'asset_set.dart';
 import 'errors.dart';
 import 'transform_logger.dart';
 import 'transform_node.dart';
-import 'utils.dart';
 
 /// Creates a [Transform] by forwarding to the private constructor.
 ///
 /// Lets [TransformNode] create [Transforms] without giving a [Transform]
 /// itself a public constructor, which would be visible to external users.
 /// Unlike the [Transform] class, this function is not exported by barback.dart.
 Transform createTransform(TransformNode node, AssetSet outputs) =>
     new Transform._(node, outputs);
 
 /// While a [Transformer] represents a *kind* of transformation, this defines
@@ skipping 38 matching lines @@
   /// A transformation can output as many assets as it wants.
   void addOutput(Asset output) {
     // TODO(rnystrom): This should immediately throw if an output with that ID
     // has already been created by this transformer.
     _outputs.add(output);
   }
 }
 
 // TODO(sigmund,rnystrom): create a separate logger for each Transfom.
 final TransformLogger _logger = new TransformLogger(true);