Returns a function that sets the tabindex on an editor instance. The returned function assumes that `this` is set to the editor instance
function setTabIndexOnEditorIframe( tabindex ) {
	return function() {
		// At this point, there's an iframe in the DOM with the id of `${editor.id}_ifr`.
		// Can't find a way to get at it without hopping out to the DOM :/
		const editorIframe = document.getElementById( `${ this.id }_ifr` );

		if ( ! editorIframe ) {
			return;
		}

		editorIframe.setAttribute( 'tabIndex', tabindex );
	};
}