Only pass an AssetId to isPrimary and declareOutputs.

pkg/polymer/lib/src/build/polyfill_injector.dart

Index: pkg/polymer/lib/src/build/polyfill_injector.dart
diff --git a/pkg/polymer/lib/src/build/polyfill_injector.dart b/pkg/polymer/lib/src/build/polyfill_injector.dart
index 6fcafd2b02adeba8a14aa6b10adedc5d69c5414d..2a3099a897090ca03c324c396a172fe0172b96ae 100644
--- a/pkg/polymer/lib/src/build/polyfill_injector.dart
+++ b/pkg/polymer/lib/src/build/polyfill_injector.dart
@@ -24,8 +24,12 @@ class PolyfillInjector extends Transformer with PolymerTransformer {
   PolyfillInjector(this.options);
 
   /// Only run on entry point .html files.
-  Future<bool> isPrimary(Asset input) =>
-      new Future.value(options.isHtmlEntryPoint(input.id));
+  // TODO(nweiz): This should just take an AssetId when barback <0.13.0 support
+  // is dropped.
+  Future<bool> isPrimary(idOrAsset) {
+    var id = idOrAsset is AssetId ? idOrAsset : idOrAsset.id;
+    return new Future.value(options.isHtmlEntryPoint(id));
+  }
 
   Future apply(Transform transform) {
     return readPrimaryAsHtml(transform).then((document) {