Reorganize barback's source files.

pkg/barback/lib/barback.dart

Index: pkg/barback/lib/barback.dart
diff --git a/pkg/barback/lib/barback.dart b/pkg/barback/lib/barback.dart
index cbb55fcf8af9c8553b5638f438a101957defdcdc..e764f1a55aad4ad84640e7bb7c78315c729febca 100644
--- a/pkg/barback/lib/barback.dart
+++ b/pkg/barback/lib/barback.dart
@@ -4,20 +4,21 @@
 
 library barback;
 
-export 'src/asset.dart';
-export 'src/asset_id.dart';
-export 'src/asset_set.dart';
+export 'src/asset/asset.dart';
+export 'src/asset/asset_id.dart';
+export 'src/asset/asset_set.dart';
 export 'src/barback.dart';
-export 'src/barback_settings.dart';
-export 'src/base_transform.dart';
 export 'src/build_result.dart';
-export 'src/declaring_transform.dart' hide DeclaringTransformController;
-export 'src/declaring_transformer.dart';
 export 'src/errors.dart' hide flattenAggregateExceptions;
-export 'src/lazy_transformer.dart';
 export 'src/log.dart';
 export 'src/package_provider.dart';
-export 'src/transform.dart' hide TransformController;
-export 'src/transform_logger.dart';
-export 'src/transformer.dart';
-export 'src/transformer_group.dart';
+export 'src/transformer/barback_settings.dart';
+export 'src/transformer/base_transform.dart';
+export 'src/transformer/declaring_transform.dart'
+    hide DeclaringTransformController;
+export 'src/transformer/declaring_transformer.dart';
+export 'src/transformer/lazy_transformer.dart';
+export 'src/transformer/transform.dart' hide TransformController;
+export 'src/transformer/transform_logger.dart';
+export 'src/transformer/transformer.dart';
+export 'src/transformer/transformer_group.dart';