Only pass an AssetId to isPrimary and declareOutputs.

pkg/polymer/pubspec.yaml

Index: pkg/polymer/pubspec.yaml
diff --git a/pkg/polymer/pubspec.yaml b/pkg/polymer/pubspec.yaml
index 1c0df78529960660261920534f78866d1dfa4a6c..597af99ed2fd3bcc86059d300a212af71e7a41de 100644
--- a/pkg/polymer/pubspec.yaml
+++ b/pkg/polymer/pubspec.yaml
@@ -9,7 +9,7 @@ homepage: https://www.dartlang.org/polymer-dart/
 dependencies:
   analyzer: '>=0.13.0 <0.14.0'
   args: '>=0.10.0 <0.11.0'
-  barback: '>=0.9.0 <0.13.0'
+  barback: '>=0.13.0 <0.14.0'
   browser: '>=0.10.0 <0.11.0'
   code_transformers: '>=0.1.0 <0.2.0'
   html5lib: '>=0.9.2 <0.10.0'