Add aggregate transformer types to barback.

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

Index: pkg/barback/lib/src/transformer/declaring_aggregate_transformer.dart
diff --git a/pkg/barback/lib/src/transformer/declaring_transformer.dart b/pkg/barback/lib/src/transformer/declaring_aggregate_transformer.dart
similarity index 59%
copy from pkg/barback/lib/src/transformer/declaring_transformer.dart
copy to pkg/barback/lib/src/transformer/declaring_aggregate_transformer.dart
index d9e5adcc54d61fc8d56b7b4998583344243437b7..a65722b029f660cbb17a27d7967a25b7dfdc7d1d 100644
--- a/pkg/barback/lib/src/transformer/declaring_transformer.dart
+++ b/pkg/barback/lib/src/transformer/declaring_aggregate_transformer.dart
@@ -2,27 +2,23 @@
 // 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.declaring_transformer;
-
-import 'dart:async';
-
-import 'declaring_transform.dart';
+library barback.transformer.declaring_aggregate_transformer;
 
 /// An interface for [Transformer]s that can cheaply figure out which assets
 /// they'll emit without doing the work of actually creating those assets.
 ///
 /// If a transformer implements this interface, that allows barback to perform
 /// optimizations to make the asset graph work more smoothly.
-abstract class DeclaringTransformer {
-  /// Declare which assets would be emitted for the primary input id specified
+abstract class DeclaringAggregateTransformer {
+  /// Declare which assets would be emitted for the primary input ids specified
   /// by [transform].
   ///
-  /// This works a little like [Transformer.apply], with two main differences.
-  /// First, instead of having access to the primary input's contents, it only
-  /// has access to its id. Second, instead of emitting [Asset]s, it just emits
-  /// [AssetId]s through [transform.addOutputId].
+  /// This works a little like [AggregateTransformer.apply], with two main
+  /// differences. First, instead of having access to the primary inputs'
+  /// contents, it only has access to their ids. Second, instead of emitting
+  /// [Asset]s, it just emits [AssetId]s through [transform.addOutputId].
   ///
   /// If this does asynchronous work, it should return a [Future] that completes
   /// once it's finished.
-  declareOutputs(DeclaringTransform transform);
+  declareOutputs(DeclaringAggregateTransform transform);
 }