Complete Batch API workaround for install system.

core/includes/install.core.inc

@@ -499,13 +499,19 @@ function install_run_task($task, &$install_state) {
     elseif ($current_batch == $function) {
       include_once DRUPAL_ROOT . '/core/includes/batch.inc';
       $output = _batch_page();
+      // Because Batch API now returns a JSON response for intermediary steps,
+      // but the installer doesn't handle Response objects yet, we will just
+      // send the output here and emulate the old model.
+      // @todo: Replace this when we refactor the installer to use a
+      // Request/Response workflow.
       if ($output instanceof Response) {
         $output->send();
+        $output = NULL;
       }
       // The task is complete when we try to access the batch page and receive
       // FALSE in return, since this means we are at a URL where we are no
       // longer requesting a batch ID.
-      elseif ($output === FALSE) {
+      if ($output === FALSE) {
         // Return nothing so the next task will run in the same request.
         variable_del('install_current_batch');
         return;