In any child theme of Boost the config icon_system standard does not work

The reason is, that \core\output\icon_system_standard does not extend icon_system. Therefore the check if the selected icon system is valid fails. \theme_config::get_icon_system checks \core\output\icon_system::is_valid_system. When icon_system_standard does not extend icon_system the check fails.

Changes to correct the bug

from:

class icon_system_standard {

to:

class icon_system_standard extends icon_system {

How to test

In a Boost child like »Classic« set 

$THEME->iconsystem = \core\output\icon_system::STANDARD;

Check the icons - they are still shown as fontawesome icons. After the patch the Moodle standard icons are shown.