Only run [declareOutputs] once for each asset/transformer pair.

pkg/barback/test/transformer/declaring_rewrite.dart

Index: pkg/barback/test/transformer/declaring_rewrite.dart
diff --git a/pkg/barback/test/transformer/lazy_rewrite.dart b/pkg/barback/test/transformer/declaring_rewrite.dart
similarity index 62%
copy from pkg/barback/test/transformer/lazy_rewrite.dart
copy to pkg/barback/test/transformer/declaring_rewrite.dart
index f694a0e77da6fd95176075593ed1be8179661b34..063ebbe1d33787667bae716049fa9a6c32f58292 100644
--- a/pkg/barback/test/transformer/lazy_rewrite.dart
+++ b/pkg/barback/test/transformer/declaring_rewrite.dart
@@ -2,7 +2,7 @@
 // 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;
+library barback.test.transformer.declaring_rewrite;
 
 import 'dart:async';
 
@@ -10,17 +10,16 @@ import 'package:barback/barback.dart';
 
 import 'rewrite.dart';
 
-/// Like [RewriteTransformer], but returns a lazy asset that doesn't perform the
-/// rewrite until it's materialized.
-class LazyRewriteTransformer extends RewriteTransformer
+/// Like [RewriteTransformer], but declares its assets ahead of time.
+class DeclaringRewriteTransformer extends RewriteTransformer
     implements LazyTransformer {
-  LazyRewriteTransformer(String from, String to)
+  DeclaringRewriteTransformer(String from, String to)
       : super(from, to);
 
   Future declareOutputs(DeclaringTransform transform) {
     if (consumePrimary) transform.consumePrimary();
     for (var extension in to.split(" ")) {
-      var id = transform.primaryInput.id.changeExtension(".$extension");
+      var id = transform.primaryId.changeExtension(".$extension");
       transform.declareOutput(id);
     }
   }