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

pkg/smoke/pubspec.yaml

Index: pkg/smoke/pubspec.yaml
diff --git a/pkg/smoke/pubspec.yaml b/pkg/smoke/pubspec.yaml
index 7d55a0e63bcfd1572833ae8da83a8078c8e68d85..e2ca45f65f2d4c918b6b550188f2461d49cd36ad 100644
--- a/pkg/smoke/pubspec.yaml
+++ b/pkg/smoke/pubspec.yaml
@@ -7,7 +7,7 @@ description: >
   can be replaced with non-reflective calls using code generation. See README.md
   for mode details.
 dependencies:
-  barback: ">=0.9.0 <0.13.0"
+  barback: ">=0.13.0 <0.14.0"
   logging: ">=0.9.0 <0.10.0"
 # TODO(sigmund): once we have some easier way to do global app-level
 # transformers, we might want to remove this transformer here and only apply it