Add aggregate transformer types to barback.

pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart

Index: pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
diff --git a/pkg/barback/lib/src/transformer/lazy_transformer.dart b/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
similarity index 59%
copy from pkg/barback/lib/src/transformer/lazy_transformer.dart
copy to pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
index 657b65b70bfbe581c8edd22d203b47f6de92de68..891fc1ec29fbf361a7aff0f6b9dfb6247bfb3a3c 100644
--- a/pkg/barback/lib/src/transformer/lazy_transformer.dart
+++ b/pkg/barback/lib/src/transformer/lazy_aggregate_transformer.dart
@@ -2,15 +2,15 @@
 // 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.transformer.lazy_transformer;
+library barback.transformer.lazy_aggregate_transformer;
 
-import 'declaring_transformer.dart';
+import 'declaring_aggregate_transformer.dart';
 
-/// An interface for [Transformer]s that indicates that the transformer's
-/// outputs shouldn't be generated until requested.
+/// An interface for [AggregateTransformer]s that indicates that the
+/// transformer's outputs shouldn't be generated until requested.
 ///
 /// The [declareOutputs] method is used to figure out which assets should be
 /// treated as "lazy." Lazy assets will only be forced to be generated if
 /// they're requested by the user or if they're used by a non-declaring
 /// transformer.
-abstract class LazyTransformer extends DeclaringTransformer {}
+abstract class LazyAggregateTransformer extends DeclaringAggregateTransformer {}