pkg/http_server: return a Future on VirtualDirectory.serveFile

dart/pkg/http_server/lib/src/virtual_directory.dart

Index: dart/pkg/http_server/lib/src/virtual_directory.dart
diff --git a/dart/pkg/http_server/lib/src/virtual_directory.dart b/dart/pkg/http_server/lib/src/virtual_directory.dart
index 6eef0b4208b3811dee2e362d8612e4fd200bea9f..117c8f1da39cbe154a9b5ee18a3ee6bca5e4f86b 100644
--- a/dart/pkg/http_server/lib/src/virtual_directory.dart
+++ b/dart/pkg/http_server/lib/src/virtual_directory.dart
@@ -171,7 +171,7 @@ class VirtualDirectory {
    * Note that if it was unabled to read from [file], the [request]s response
    * is closed with error-code [HttpStatus.NOT_FOUND].
    */
-  void serveFile(File file, HttpRequest request) {
+  Future serveFile(File file, HttpRequest request) {
     var response = request.response;
     // TODO(ajohnsen): Set up Zone support for these errors.
     file.lastModified().then((lastModified) {
@@ -238,6 +238,8 @@ class VirtualDirectory {
       response.statusCode = HttpStatus.NOT_FOUND;
       response.close();
     });
+    
+    return response.done;
   }
 
   void _serveDirectory(Directory dir, HttpRequest request) {