Issue #1377740: revert unintentional theme.inc changes.

core/includes/theme.inc

@@ -134,8 +134,7 @@ function drupal_theme_rebuild() {
  */
 function drupal_find_theme_functions($cache, $prefixes) {
   $implementations = [];
-  $functions = get_defined_functions();
-  $theme_functions = preg_grep('/^(' . implode(')|(', $prefixes) . ')_/', $functions['user']);
+  $grouped_functions = \Drupal::service('theme.registry')->getPrefixGroupedUserFunctions();
 
   foreach ($cache as $hook => $info) {
     foreach ($prefixes as $prefix) {
@@ -151,8 +150,10 @@ function drupal_find_theme_functions($cache, $prefixes) {
       // are found using the base hook's pattern, not a pattern from an
       // intermediary suggestion.
       $pattern = isset($info['pattern']) ? $info['pattern'] : ($hook . '__');
-      if (!isset($info['base hook']) && !empty($pattern)) {
-        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $theme_functions);
+      // Grep only the functions which are within the prefix group.
+      list($first_prefix,) = explode('_', $prefix, 2);
+      if (!isset($info['base hook']) && !empty($pattern) && isset($grouped_functions[$first_prefix])) {
+        $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $grouped_functions[$first_prefix]);
         if ($matches) {
           foreach ($matches as $match) {
             $new_hook = substr($match, strlen($prefix) + 1);