Here's a trick from HTML5 Boilerplate for Jquery. I presume it could be modified for UI (or any library).
<script
src="//ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.min.js"></script>
<script>window.jQuery || document.write("<script
src='{{=URL('static','js/jquery.js')}}'>\x3C/script>")</script>
