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

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

Index: pkg/polymer/lib/src/build/script_compactor.dart
diff --git a/pkg/polymer/lib/src/build/script_compactor.dart b/pkg/polymer/lib/src/build/script_compactor.dart
index dfb25cf5f66e1a42ddfa71701b93696c823c182d..61cec74a2bae4980ad047b4ba989fcec307de68e 100644
--- a/pkg/polymer/lib/src/build/script_compactor.dart
+++ b/pkg/polymer/lib/src/build/script_compactor.dart
@@ -49,8 +49,8 @@ class ScriptCompactor extends Transformer {
       : resolvers = new Resolvers(sdkDir != null ? sdkDir : dartSdkDirectory);
 
   /// Only run on entry point .html files.
-  Future<bool> isPrimary(Asset input) =>
-      new Future.value(options.isHtmlEntryPoint(input.id));
+  Future<bool> isPrimary(AssetId id) =>
+      new Future.value(options.isHtmlEntryPoint(id));
 
   Future apply(Transform transform) =>
       new _ScriptCompactor(transform, options, resolvers).apply();