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

pkg/polymer/lib/src/transform/script_compactor.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 combines multiple dart script tags into a single one. */
 library polymer.src.transform.script_compactor;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
@@ skipping 44 matching lines @@
         if (libraryId == null) continue;
         libraries.add(libraryId);
       }
 
       if (!changed) {
         transform.addOutput(new Asset.fromString(id, content));
         return;
       }
 
       var bootstrapId = id.addExtension('_bootstrap.dart');
-      var filename = path.basename(bootstrapId.path);
+      var filename = path.url.basename(bootstrapId.path);
       document.body.nodes.add(parseFragment(
             '<script type="application/dart" src="$filename"></script>'));
 
       var urls = libraries.map((id) => importUrlFor(id, bootstrapId, logger))
           .where((url) => url != null).toList();
       var buffer = new StringBuffer()..write(_header);
       for (int i = 0; i < urls.length; i++) {
         buffer.writeln("import '${urls[i]}' as i$i;");
       }
       buffer..write(_mainPrefix)
@@ skipping 14 matching lines @@
     if (id.path.startsWith('lib/')) {
       return 'package:${id.package}/${id.path.substring(4)}';
     }
     
     // Use relative urls only if it's possible.
     if (id.package != sourceId.package) {
       logger.error("don't know how to import $id from $sourceId");
       return null;
     }
 
-    var upPath = path.joinAll(path.split(sourceId.path).map((_) => '..'));
-    return path.normalize(path.join(sourceId.path, upPath, id.path));
+    var urlBuilder = path.url;
+    var upPath = urlBuilder.joinAll(
+        urlBuilder.split(sourceId.path).map((_) => '..'));
+    return urlBuilder.normalize(
+        urlBuilder.join(sourceId.path, upPath, id.path));
   }
 }
 
 const _header = """
 library app_bootstrap;
 
 import 'package:polymer/polymer.dart';
 import 'dart:mirrors' show currentMirrorSystem;
 
 """;
 
 const _mainPrefix = """
 
 void main() {
   initPolymer([
 """;
 
 // TODO(sigmund): investigate alternative to get the baseUri (dartbug.com/12612)
 const _mainSuffix = """
     ], currentMirrorSystem().isolate.rootLibrary.uri.toString());
 }
 """;