Since rMWVD46bc943c1616 , mediawiki/vendor REL1_31 branch no more includes dependencies for WMF deployed extensions. It is restricted to packages used for the MediaWiki tarball.
Hence extensions not being part of the tarball would fail the CI tests on REL1_31 whenever they have a dependency that is not listed.
Examples:
AbuseFilter: PHP Fatal Error: Class 'Wikimedia\EquivSet\EquivSet' not found
[12-Mar-2018 16:01:15 UTC] PHP Fatal error: Class 'Wikimedia\Equivset\Equivset' not found in /var/www/core/extensions/AbuseFilter/includes/parser/AbuseFilterParser.php on line 1266 2018-03-12 16:01:15 language-cx2 my_wiki: [e4d91a1e] PHP Fatal Error: Class 'Wikimedia\Equivset\Equivset' not found
Thanks REL1_31 tests failing due to unknown class Pimple\Container
20:30:55 Fatal error: unknown class Pimple\Container in /workspace/src/extensions/Flow/includes/Container.php on line 5
I've run composer update to fetch latest dependencies.