Fix a deadlock bug with lazy transformer pipelining.

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 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:barback/src/utils.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)
+  DeclareAssetsTransformer(Iterable<String> declared, [Iterable<String> emitted])
       : this.declared = declared.map((id) => new AssetId.parse(id)).toList(),
-        this.emitted = emitted.map((id) => new AssetId.parse(id)).toList();
+        this.emitted = (emitted == null ? declared : emitted)
+            .map((id) => new AssetId.parse(id)).toList();
 
   Future<bool> doIsPrimary(AssetId id) => new Future.value(true);
 
   Future doApply(Transform transform) {
     return newFuture(() {
       for (var id in emitted) {
         transform.addOutput(new Asset.fromString(id, id.toString()));
       }
     });
   }
 
   Future declareOutputs(DeclaringTransform transform) {
     return newFuture(() {
       declared.forEach(transform.declareOutput);
     });
   }
 }