Reorganize barback's source files.

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 'package:stack_trace/stack_trace.dart';
 
-import 'asset_id.dart';
-import 'transformer.dart';
+import 'asset/asset_id.dart';
+import 'transformer/transformer.dart';
 import 'utils.dart';
 
 /// Error thrown when an asset with [id] cannot be found.
 class AssetNotFoundException implements Exception {
   final AssetId id;
 
   AssetNotFoundException(this.id);
 
   String toString() => "Could not find asset $id.";
 }
@@ skipping 155 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";
 }