Expose aggregate transformers in barback.

pkg/barback/test/transformer/declare_assets.dart

 // Copyright (c) 2013, 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.declare_asset;
 
 import 'package:barback/barback.dart';
 
 import 'mock.dart';
 
 /// A transformer that declares some outputs and emits others.
 class DeclareAssetsTransformer extends MockTransformer
     implements DeclaringTransformer {
   /// The assets that the transformer declares that it will emit.
   final List<AssetId> declared;
 
   /// The assets that the transformer actually emits.
   ///
   /// These assets' contents will be identical to their ids.
   final List<AssetId> emitted;
 
-  DeclareAssetsTransformer(Iterable<String> declared, [Iterable<String> emitted])
+  /// If this is non-`null`, assets are only declared for this input.
+  final AssetId input;
+
+  DeclareAssetsTransformer(Iterable<String> declared, {Iterable<String> emitted,
+        String input})
       : this.declared = declared.map((id) => new AssetId.parse(id)).toList(),
         this.emitted = (emitted == null ? declared : emitted)
-            .map((id) => new AssetId.parse(id)).toList();
+            .map((id) => new AssetId.parse(id)).toList(),
+        this.input = input == null ? null : new AssetId.parse(input);
 
-  bool doIsPrimary(AssetId id) => true;
+  bool doIsPrimary(AssetId id) => input == null || id == input;
 
   void doApply(Transform transform) {
     for (var id in emitted) {
       transform.addOutput(new Asset.fromString(id, id.toString()));
     }
   }
 
   void declareOutputs(DeclaringTransform transform) {
     declared.forEach(transform.declareOutput);
   }
 }