src/vs/workbench/api/common/menusExtensionPoint.ts/isValidIcon
	function isValidIcon(icon: IUserFriendlyIcon | undefined, collector: ExtensionMessageCollector): boolean {
		if (typeof icon === 'undefined') {
			return true;
		}
		if (typeof icon === 'string') {
			return true;
		} else if (typeof icon.dark === 'string' && typeof icon.light === 'string') {
			return true;
		}
		collector.error(localize('opticon', "property `icon` can be omitted or must be either a string or a literal like `{dark, light}`"));
		return false;
	}