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

pkg/barback/test/transformer/lazy_rewrite.dart

Index: pkg/barback/test/transformer/lazy_rewrite.dart
diff --git a/pkg/barback/test/transformer/lazy_rewrite.dart b/pkg/barback/test/transformer/lazy_rewrite.dart
index f694a0e77da6fd95176075593ed1be8179661b34..33a92cd16b742c1f20273c3159b91697c757af04 100644
--- a/pkg/barback/test/transformer/lazy_rewrite.dart
+++ b/pkg/barback/test/transformer/lazy_rewrite.dart
@@ -8,20 +8,12 @@ import 'dart:async';
 
 import 'package:barback/barback.dart';
 
-import 'rewrite.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 RewriteTransformer
+class LazyRewriteTransformer extends DeclaringRewriteTransformer
     implements LazyTransformer {
   LazyRewriteTransformer(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");
-      transform.declareOutput(id);
-    }
-  }
 }