Apply several phases only to entrypoint files (which currently are identified as

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

Index: pkg/polymer/lib/src/transform/polyfill_injector.dart
diff --git a/pkg/polymer/lib/src/transform/polyfill_injector.dart b/pkg/polymer/lib/src/transform/polyfill_injector.dart
index 899fdd81d72c725b6666e918f04a7240a40e182a..6e13b9007725e7c8529600f18f5fc8c009ae8b33 100644
--- a/pkg/polymer/lib/src/transform/polyfill_injector.dart
+++ b/pkg/polymer/lib/src/transform/polyfill_injector.dart
@@ -22,8 +22,8 @@ import 'common.dart';
  * css shimming).
  */
 class PolyfillInjector extends Transformer {
-  /** Only run this transformer on .html files. */
-  final String allowedExtensions = ".html";
+  /** Only run on entrypoint .html files under web or test. */
+  Future<bool> isPrimary(Asset input) => isHtmlInWebOrTest(input.id);
 
   Future apply(Transform transform) {
     var id = transform.primaryInput.id;