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

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

Index: pkg/polymer/lib/src/build/import_inliner.dart
diff --git a/pkg/polymer/lib/src/build/import_inliner.dart b/pkg/polymer/lib/src/build/import_inliner.dart
index f565068f69110ffe2959b4bda4763ec708e4e60f..73163bde42dac73aac20dace0f8c5084c068f37e 100644
--- a/pkg/polymer/lib/src/build/import_inliner.dart
+++ b/pkg/polymer/lib/src/build/import_inliner.dart
@@ -244,8 +244,8 @@ class ImportInliner extends Transformer {
   ImportInliner(this.options);
 
   /// 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 _HtmlInliner(options, transform).apply();