Expose aggregate transformers in barback.

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 c39646a0bc649236dc377dd643c9d053a5b752df..3975758597c546d0e7de8bf0b65becb705ea95b4 100644
--- a/pkg/barback/lib/barback.dart
+++ b/pkg/barback/lib/barback.dart
@@ -12,10 +12,15 @@ export 'src/build_result.dart';
 export 'src/errors.dart' hide flattenAggregateExceptions;
 export 'src/log.dart';
 export 'src/package_provider.dart';
+export 'src/transformer/aggregate_transform.dart';
+export 'src/transformer/aggregate_transformer.dart';
 export 'src/transformer/barback_settings.dart';
 export 'src/transformer/base_transform.dart';
+export 'src/transformer/declaring_aggregate_transform.dart';
+export 'src/transformer/declaring_aggregate_transformer.dart';
 export 'src/transformer/declaring_transform.dart' hide newDeclaringTransform;
 export 'src/transformer/declaring_transformer.dart';
+export 'src/transformer/lazy_aggregate_transformer.dart';
 export 'src/transformer/lazy_transformer.dart';
 export 'src/transformer/transform.dart' hide newTransform;
 export 'src/transformer/transform_logger.dart';