Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/lazy_rewrite.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_rewrite;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 
 import 'declaring_rewrite.dart';
 
 /// Like [RewriteTransformer], but returns a lazy asset that doesn't perform the
 /// rewrite until it's materialized.
 class LazyRewriteTransformer extends DeclaringRewriteTransformer
     implements LazyTransformer {
   LazyRewriteTransformer(String from, String to)
       : super(from, to);
 }