Menu tree context alter

menu-tree.vars.php

<?php
/**
 * Implements hook_theme_registry_alter().
 */
function THEMENAME_theme_registry_alter(&$theme_registry) {
  array_unshift($theme_registry['menu_tree']['preprocess functions'], 'THEMENAME_menu_tree_alter');
}

/**
 * Helper function for adding the tree context to the variables.
 *
 * A faux "hook_menu_tree_alter()" if you will.
 */
function THEMENAME_menu_tree_alter(&$variables) {
  // Save the original tree render array so it can be
  // used in future theme preprocessors.
  $variables['#tree'] = $variables['tree'];
}