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

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

Index: pkg/polymer/lib/src/build/build_filter.dart
diff --git a/pkg/polymer/lib/src/build/build_filter.dart b/pkg/polymer/lib/src/build/build_filter.dart
index fc2138d72e654eb485e0deafe7b9d20850b8dd34..8e73a4b8f93e5e052f9d22dde0e7b5ea32f40771 100644
--- a/pkg/polymer/lib/src/build/build_filter.dart
+++ b/pkg/polymer/lib/src/build/build_filter.dart
@@ -17,16 +17,16 @@ class BuildFilter extends Transformer with PolymerTransformer {
   final TransformOptions options;
   BuildFilter(this.options);
 
-  Future<bool> isPrimary(Asset input) => new Future.value(
+  Future<bool> isPrimary(AssetId id) => new Future.value(
       // nothing is filtered in debug mode
       options.releaseMode &&
       // TODO(sigmund): remove this exclusion once we have dev_transformers
       // (dartbug.com/14187)
-      input.id.path.startsWith('web/') &&
+      id.path.startsWith('web/') &&
       // may filter non-entry HTML files and internal artifacts
-      (input.id.extension == '.html' || input.id.extension == '.scriptUrls') &&
+      (id.extension == '.html' || id.extension == '.scriptUrls') &&
       // keep any entry points
-      !options.isHtmlEntryPoint(input.id));
+      !options.isHtmlEntryPoint(id));
 
   Future apply(Transform transform) {
     if (transform.primaryInput.id.extension == '.scriptUrls') {