Only pass an AssetId to isPrimary and declareOutputs.

pkg/polymer/lib/src/build/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.
 
 /// Includes any additional polyfills that may needed by the deployed app.
 library polymer.src.build.polyfill_injector;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
 import 'package:html5lib/dom.dart' show
     Document, DocumentFragment, Element, Node;
 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 polyfills and interop.js (used for css shimming).
 ///
 /// This step also replaces "packages/browser/dart.js" and the Dart script tag
 /// with a script tag that loads the dart2js compiled code directly.
 class PolyfillInjector extends Transformer with PolymerTransformer {
   final TransformOptions options;
 
   PolyfillInjector(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) {
     return readPrimaryAsHtml(transform).then((document) {
       bool webComponentsFound = false;
       bool jsInteropFound = false;
       Element dartJs;
       final dartScripts = <Element>[];
 
       for (var tag in document.querySelectorAll('script')) {
         var src = tag.attributes['src'];
@@ skipping 60 matching lines @@
       }
 
       transform.addOutput(
           new Asset.fromString(transform.primaryInput.id, document.outerHtml));
     });
   }
 }
 
 final _webComponentsJS = new RegExp(r'platform.*\.js',
     caseSensitive: false);