Reorganize barback's source files.

pkg/barback/lib/src/transformer/transform_logger.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_logger;
+library barback.transformer.transform_logger;
 
 import 'package:source_maps/span.dart';
 
-import 'asset_id.dart';
-import 'log.dart';
+import '../asset/asset_id.dart';
+import '../log.dart';
 
 typedef void LogFunction(AssetId asset, LogLevel level, String message,
                          Span span);
 
 /// Object used to report warnings and errors encountered while running a
 /// transformer.
 class TransformLogger {
   final LogFunction _logFunction;
 
   TransformLogger(this._logFunction);
@@ skipping 41 matching lines @@
   /// primary input nor any outputs emitted by the transformer will be passed on
   /// to the following phase, and the build will be reported as having failed.
   ///
   /// Unlike throwing an exception, this doesn't cause a transformer to stop
   /// running. This makes it useful in cases where a single input may have
   /// multiple errors that the user wants to know about.
   void error(String message, {AssetId asset, Span span}) {
     _logFunction(asset, LogLevel.ERROR, message, span);
   }
 }