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

pkg/barback/test/transformer/catch_asset_not_found.dart

Index: pkg/barback/test/transformer/catch_asset_not_found.dart
diff --git a/pkg/barback/test/transformer/catch_asset_not_found.dart b/pkg/barback/test/transformer/catch_asset_not_found.dart
index 2e4e24b5912a1fd4070d44c201ab7e7d04d962dd..ee5371dff1e07927feddcf3ab8c23ef850749a70 100644
--- a/pkg/barback/test/transformer/catch_asset_not_found.dart
+++ b/pkg/barback/test/transformer/catch_asset_not_found.dart
@@ -23,8 +23,8 @@ class CatchAssetNotFoundTransformer extends MockTransformer {
   CatchAssetNotFoundTransformer(this.extension, String input)
       : input = new AssetId.parse(input);
 
-  Future<bool> doIsPrimary(Asset asset) =>
-      new Future.value(asset.id.extension == extension);
+  Future<bool> doIsPrimary(AssetId id) =>
+      new Future.value(id.extension == extension);
 
   Future doApply(Transform transform) {
     return transform.getInput(input).then((_) {