A new test for missing image files referenced from CSS I'm adding in https://gerrit.wikimedia.org/r/#/c/235903/ exposed an issue:
https://integration.wikimedia.org/ci/job/mediawiki-extensions-hhvm/30048/consoleFull:
00:50:20 3) ResourcesTest::testFileExistence with data set #1260 ('/mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/modules/styles/flow/icons/ajax-loader.gif', 'ext.flow.ui', '/mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/modules/styles/flow/icons/ajax-loader.gif') 00:50:20 File '/mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/modules/styles/flow/icons/ajax-loader.gif' referenced by 'ext.flow.ui' must exist. 00:50:20 Failed asserting that file "/mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/extensions/Flow/modules/styles/flow/icons/ajax-loader.gif" exists. 00:50:20 00:50:20 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/structure/ResourcesTest.php:24 00:50:20 /mnt/jenkins-workspace/workspace/mediawiki-extensions-hhvm/src/tests/phpunit/MediaWikiTestCase.php:131
You could probably be using jquery.spinner instead?