Toolbar menu theme override omits the 'menu_name' variable

Problem/Motivation

The Toolbar module is providing a theme override for the menu theme hook: menu__toolbar. This override does not provide all the variables from the base hook, which can cause the following notice in preprocess functions:

Notice: Undefined index: menu_name

Historically, this was caused during the early days of Drupal 8.0.x, when #2609400: menu.html.twig says that menu_name is an available variable, but it's not introduced the menu_name variable, but it was not realized at the time that this theme hook was overridden in the Toolbar module.

This is a minor issue because the notice only occurs when using a theme which does not provide a menu--toolbar.html.twig template. The core stable theme includes this, so all themes that extend the stable theme are not affected.

Steps to reproduce

Use a theme which does not use the stable base theme. The menu_name variable will not be available for the Toolbar menus.

Proposed resolution

Declare the menu_name variable in the theme override, so it is identical to the base theme hook that is declared in drupal_common_theme().

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.

Release notes snippet

N/A