Add aggregate transformer types to barback.

pkg/barback/lib/barback.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;
 
 export 'src/asset/asset.dart';
 export 'src/asset/asset_id.dart';
 export 'src/asset/asset_set.dart';
 export 'src/barback.dart';
 export 'src/build_result.dart';
 export 'src/errors.dart' hide flattenAggregateExceptions;
 export 'src/log.dart';
 export 'src/package_provider.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_transform.dart' hide newDeclaringTransform;
 export 'src/transformer/declaring_transformer.dart';
 export 'src/transformer/lazy_transformer.dart';
-export 'src/transformer/transform.dart' hide TransformController;
+export 'src/transformer/transform.dart' hide newTransform;
 export 'src/transformer/transform_logger.dart';
 export 'src/transformer/transformer.dart';
 export 'src/transformer/transformer_group.dart';