Expose aggregate transformers in barback.

pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart

Index: pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart
diff --git a/pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart b/pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart
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index 0000000000000000000000000000000000000000..237734514650ae6ed210f70d1a6bd224145e0f61
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+++ b/pkg/barback/test/transformer/declaring_aggregate_many_to_many.dart
@@ -0,0 +1,23 @@
+// 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_many;
+
+import 'dart:async';
+
+import 'package:barback/barback.dart';
+
+import 'aggregate_many_to_many.dart';
+
+/// Like [AggregateManyToManyTransformer], but declares its assets ahead of
+/// time.
+class DeclaringAggregateManyToManyTransformer
+    extends AggregateManyToManyTransformer
+    implements DeclaringAggregateTransformer {
+  DeclaringAggregateManyToManyTransformer(String extension)
+      : super(extension);
+
+  Future declareOutputs(DeclaringAggregateTransform transform) =>

[Bob Nystrom, 2014/05/27 17:20:18]

Put the type parameter in the return type (if you can?).

[nweiz, 2014/05/27 21:03:10]

I don't want to do that, because the specific return value isn't relevant to the
function's signature. The only thing that's relevant to the caller is that the
future will complete when the function is done declaring its outputs.

[Bob Nystrom, 2014/05/28 21:03:58]

Okey-dokey.

+      transform.primaryIds.asyncMap(transform.declareOutput).toList();
+}