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

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

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /** Common methods used by transfomers. */
 library polymer.src.transform.common;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 54 matching lines @@
     package = segments[1];
     targetPath = urlBuilder.join('asset',
         urlBuilder.joinAll(segments.sublist(2)));
   } else {
     package = source.package;
     targetPath = urlBuilder.normalize(
         urlBuilder.join(urlBuilder.dirname(source.path), url));
   }
   return new AssetId(package, targetPath);
 }
+
+/** Whether an asset with [id] is considered a primary entry point HTML file. */
+Future<bool> isPrimaryHtml(AssetId id) =>
+    new Future.value(id.extension == '.html' &&
+        // Note: [id.path] is a relative path from the root of a package.
+        (id.path.startsWith('web/') || id.path.startsWith('test/')));