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

pkg/polymer/lib/src/transform/polyfill_injector.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.
 
 /**
  * Final phase of the polymer transformation: includes any additional polyfills
  * that may needed by the deployed app.
  */
 library polymer.src.transform.polyfill_injector;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:html5lib/dom.dart' show Document, Node, DocumentFragment;
 import 'package:html5lib/parser.dart' show parseFragment;
 import 'common.dart';
 
 /**
  * Ensures that any scripts and polyfills needed to run a polymer application
  * are included. For example, this transformer will ensure that there is a
  * script tag that loads the shadow_dom polyfill and interop.js (used for the
  * css shimming).
  */
 class PolyfillInjector extends Transformer {
-  /** Only run this transformer on .html files. */
-  final String allowedExtensions = ".html";
+  /** Only run on entry point .html files. */
+  Future<bool> isPrimary(Asset input) => isPrimaryHtml(input.id);
 
   Future apply(Transform transform) {
     var id = transform.primaryInput.id;
     return transform.primaryInput.readAsString().then((content) {
-      var document = parseHtml(content, id.path, transform.logger);
+      var document = parseHtml(content, id.path, transform.logger,
+          checkDocType: false);
       bool shadowDomFound = false;
       bool jsInteropFound = false;
       bool dartScriptTags = false;
 
       for (var tag in document.queryAll('script')) {
         var src = tag.attributes['src'];
         if (src != null) {
           var last = src.split('/').last;
           if (last == 'interop.js') {
             jsInteropFound = true;
@@ skipping 25 matching lines @@
         document.body.nodes.insert(0, parseFragment(
             '<script src="packages/shadow_dom/shadow_dom.min.js"></script>\n'));
       }
 
       transform.addOutput(new Asset.fromString(id, document.outerHtml));
     });
   }
 }
 
 final _shadowDomJS = new RegExp(r'shadow_dom\..*\.js', caseSensitive: false);