Although diff of js2stopgap.js show no changes at all (CodeReview bug?), comparison between what was before this change: <http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/js2/js2stopgap.js?view=markup&pathrev=58093> with version after <http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/js2/js2stopgap.js?view=markup&pathrev=58959> indicates that it introduced some unneeded changes to regexps, thus causing bug 21709 that had to be fixed by r58997, r59666 and r59671.
"on non-lib non-minimised JavaScript files in js2"
Somehow this did change js2stopgap.js , which is an external library. Will revert change to those files in an hour.