Fix a deadlock bug with lazy transformer pipelining.

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 'dart:async';
+
+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);
+}