Steps to Reproduce:

When attempting to add OOUI icons to buttons using JavaScript for a user script, I add the code:

var rollbackbutton = new OO.ui.ButtonWidget({
   autosize: true,
   label: 'Rollback',
   flags: [
      'primary',
      'progressive'
   ],
   icon: "editUndo",
   align: 'inline'
});
$(selector).append(rollbackbutton.$element);

in JavaScript, using the appropriate icon code from https://doc.wikimedia.org/oojs-ui/master/demos/?page=icons

Actual Results:

What happens is that the button loads with no icon at all. I double-checked my code for errors, and there are no errors whatsoever.

Expected Results:

The button should load with the icon "editUndo" next to it.