src/vs/css.ts/unbind
function attachListeners(name: string, linkNode: HTMLLinkElement, callback: () => void, errorback: (err: any) => void): void {
	const unbind = () => {
		linkNode.removeEventListener('load', loadEventListener);
		linkNode.removeEventListener('error', errorEventListener);
	};
	const loadEventListener = (e: any) => {
		unbind();
		callback();
	};
	const errorEventListener = (e: any) => {
		unbind();
		errorback(e);
	};
	linkNode.addEventListener('load', loadEventListener);
	linkNode.addEventListener('error', errorEventListener);
}