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

pkg/barback/lib/src/errors.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.errors;
 
 import 'dart:async';
-import 'dart:io';
 
 import 'package:stack_trace/stack_trace.dart';
 
 import 'asset_id.dart';
 import 'transformer.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
   final AssetId id;
 
@@ skipping 95 matching lines @@
 
   bool operator==(other) =>
       other is TransformInfo &&
       other.transformer == transformer &&
       other.primaryId == primaryId;
 
   int get hashCode => transformer.hashCode ^ primaryId.hashCode;
 
   String toString() => "$transformer on $primaryId";
 }