material_design_icons_flutter: no icons are rendered on screen

Question

No icons are rendered on screen for icon names including:

person_add 
language 
schedule
dark_mode

material_design_icons_flutter: 7.0.7296

Icon(MdiIcons.fromString(iconName))

e.g.,

MenuItemButton(
  leadingIcon: Icon(MdiIcons.fromString('person_add')),
  child: Text('Click Me'),
);