Expose aggregate transformers in barback.

pkg/barback/test/transformer/lazy_assets.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.test.transformer.lazy_assets;
 
 import 'package:barback/barback.dart';
 
 import 'declare_assets.dart';
 
 /// Like [DeclareAssetsTransformer], but lazy.
 class LazyAssetsTransformer extends DeclareAssetsTransformer
     implements LazyTransformer {
-  LazyAssetsTransformer(Iterable<String> declared, [Iterable<String> emitted])
-      : super(declared, emitted);
+  LazyAssetsTransformer(Iterable<String> declared, {Iterable<String> emitted,
+        String input})
+      : super(declared, emitted: emitted, input: input);
 }