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

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

Index: pkg/polymer/lib/src/transform/code_extractor.dart
diff --git a/pkg/polymer/lib/src/transform/code_extractor.dart b/pkg/polymer/lib/src/transform/code_extractor.dart
index 14ff6522c24422209b96626f8ac36687a6c1ce97..5f7058f01974f4dc335fbe5a05e797e577f45925 100644
--- a/pkg/polymer/lib/src/transform/code_extractor.dart
+++ b/pkg/polymer/lib/src/transform/code_extractor.dart
@@ -30,7 +30,7 @@ class InlineCodeExtractor extends Transformer {
             !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');