Fix polymer transformers in windows. AssetId's always use posix style paths

pkg/polymer/lib/src/transform/code_extractor.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.
 
 /** Transfomer that extracts inlined script code into separate assets. */
 library polymer.src.transformers;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 12 matching lines @@
   Future apply(Transform transform) {
     var inputId = transform.primaryId;
     return getPrimaryContent(transform).then((content) {
       var document = parseHtml(content, inputId.path, transform.logger);
       int count = 0;
       for (var tag in document.queryAll('script')) {
         if (tag.attributes['type'] == 'application/dart' &&
             !tag.attributes.containsKey('src')) {
           // TODO(sigmund): should we automatically include a library directive
           // if it doesn't have one?
-          var filename = path.basename(inputId.path);
+          var filename = path.url.basename(inputId.path);
           tag.attributes['src'] = '$filename.$count.dart';
           var textContent = tag.nodes.first;
           var id = inputId.addExtension('.$count.dart');
           transform.addOutput(new Asset.fromString(id, textContent.value));
           textContent.remove();
           count++;
         }
       }
       transform.addOutput(new Asset.fromString(inputId,
           count == 0 ? content : document.outerHtml));
     });
   }
 }