Add aggregate transformer types to barback.

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

+// Copyright (c) 2014, 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.transformer.declaring_aggregate_transformer;
+
+import 'declaring_aggregate_transform.dart';
+
+/// 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 DeclaringAggregateTransformer {
+  /// Declare which assets would be emitted for the primary input ids specified
+  /// by [transform].
+  ///
+  /// 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.
+  ///
+  /// This may complete before [DeclaringAggregateTransform.primaryIds] stream
+  /// is closed. For example, it may know that each key will only have two
+  /// inputs associated with it, and so use `transform.primaryIds.take(2)` to
+  /// access only those inputs' ids.
+  declareOutputs(DeclaringAggregateTransform transform);
+}