Issue #1659000 by sun: Fixed $form_id is not recorded/provided in $form_state['build_info'].

includes/form.inc

@@ -164,6 +164,8 @@ function drupal_get_form($form_id) {
  *       automatically loaded by form_get_cache(). By default the current menu
  *       router item's 'file' definition is added, if any. Use
  *       form_load_include() to add include files from a form constructor.
+ *     - form_id: Identification of the primary form being constructed and
+ *       processed.
  *     - base_form_id: Identification for a base form, as declared in a
  *       hook_forms() implementation.
  *   - rebuild_info: Internal. Similar to 'build_info', but pertaining to
@@ -721,6 +723,9 @@ function drupal_form_submit($form_id, &$form_state) {
 function drupal_retrieve_form($form_id, &$form_state) {
   $forms = &drupal_static(__FUNCTION__);
 
+  // Record the $form_id.
+  $form_state['build_info']['form_id'] = $form_id;
+
   // Record the filepath of the include file containing the original form, so
   // the form builder callbacks can be loaded when the form is being rebuilt
   // from cache on a different path (such as 'system/ajax'). See