We have not used YUI controls in mojoPortal for a long time so we removed some of those files. In the repository the Web\Controls\YUI folder has been excluded from the mojoPortal.Web project and the files have been removed. So it sounds like you either don't have the latest version from the repository or somehow your copy of the project file did not exclude those files. You just need to right click the /Controls/YUI folder in Solution Explorer and choose exclude files. If you get any other similar errors just find the reference to the missing file and remove it.
Hope that helps,