Expose aggregate transformers in barback.

pkg/barback/test/transformer/declaring_aggregate_many_to_one.dart

Index: pkg/barback/test/transformer/declaring_aggregate_many_to_one.dart
diff --git a/pkg/barback/test/transformer/declaring_aggregate_many_to_one.dart b/pkg/barback/test/transformer/declaring_aggregate_many_to_one.dart
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+// 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.test.transformer.declaring_aggregate_many_to_one;
+
+import 'package:barback/barback.dart';
+import 'package:path/path.dart' as path;
+
+import 'aggregate_many_to_one.dart';
+
+/// Like [AggregateManyToOneTransformer], but declares its assets ahead of time.
+class DeclaringAggregateManyToOneTransformer
+    extends AggregateManyToOneTransformer
+    implements DeclaringAggregateTransformer {
+  DeclaringAggregateManyToOneTransformer(String extension, String output)
+      : super(extension, output);
+
+  void declareOutputs(DeclaringAggregateTransform transform) {
+    transform.declareOutput(new AssetId(transform.package,
+        path.url.join(transform.key, output)));
+  }
+}