In the live help for FileRepoInfo, the apiurl property shows up as always available, but in reality, it's only available for wikis with InstantCommons enabled and will spit out a warning message on any other wiki. (Compare the example from the help on: MediaWiki vs RationalWiki.)
The problem is that apiurl is hard-coded into the apihelp-query+filerepoinfo-param-prop entry in the help file (includes/api/i18n/en.json) as well as being used in the example in incudes/api/ApiQueryFileRepoInfo.php.