Reorganize barback's source files.

pkg/barback/lib/src/log.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.log;
 
 import 'package:source_maps/span.dart';
 
-import 'asset_id.dart';
+import 'asset/asset_id.dart';
 import 'errors.dart';
 
 /// The severity of a logged message.
 class LogLevel {
   static const INFO = const LogLevel("Info");
   static const FINE = const LogLevel("Fine");
   static const WARNING = const LogLevel("Warning");
   static const ERROR = const LogLevel("Error");
 
   final String name;
@@ skipping 12 matching lines @@
 
   final LogLevel level;
   final String message;
 
   /// The location that the message pertains to or null if not associated with
   /// a source [Span].
   final Span span;
 
   LogEntry(this.transform, this.assetId, this.level, this.message, this.span);
 }