Don't require Transformer methods to return Futures.

pkg/barback/test/transformer/declaring_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.declaring_rewrite;
 
-import 'dart:async';
-
 import 'package:barback/barback.dart';
 
 import 'rewrite.dart';
 
 /// Like [RewriteTransformer], but declares its assets ahead of time.
 class DeclaringRewriteTransformer extends RewriteTransformer
     implements DeclaringTransformer {
   DeclaringRewriteTransformer(String from, String to)
       : super(from, to);
 
-  Future declareOutputs(DeclaringTransform transform) {
+  void declareOutputs(DeclaringTransform transform) {
     if (consumePrimary) transform.consumePrimary();
     for (var extension in to.split(" ")) {
       var id = transform.primaryId.changeExtension(".$extension");
       transform.declareOutput(id);
     }
   }
 }