Don't require Transformer methods to return Futures.

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);
 }