pkg/http_server: return future for VirtualDirectory serveRequest

pkg/http_server/test/virtual_directory_test.dart

Index: pkg/http_server/test/virtual_directory_test.dart
diff --git a/pkg/http_server/test/virtual_directory_test.dart b/pkg/http_server/test/virtual_directory_test.dart
index eff53cccaa3ead1b0e299c287ff01b1936e12efd..7be2a1251d1045e12736d512749609a16b7fe5c5 100644
--- a/pkg/http_server/test/virtual_directory_test.dart
+++ b/pkg/http_server/test/virtual_directory_test.dart
@@ -192,9 +192,11 @@ void main() {
         new Directory('${dir.path}/dir').createSync();
         var virDir = new VirtualDirectory(dir.path);
         virDir.allowDirectoryListing = true;
-        virDir.directoryHandler = (dir2, request) {
+
+        virDir.directoryHandler = protectAsync2((dir2, request) {
           fail('not expected');
-        };
+        });
+
         return getStatusCodeForVirtDir(virDir, '/dir', followRedirects: false)
           .then((result) {
             expect(result, 301);