While working on
MDL-57510. I realized that -
If I create \theme_boost\output\core_question_renderer, everything works as expected. But \theme_boost\output\core\question_renderer doesn't work (default renderer gets called instead)
This was happening because I was calling $PAGE->get_renderer("core_question") instead of get_renderer("core", "question")
Look at lib/outputfactories.php, theme_overridden_renderer_factory, to know why it makes a difference.
Upon looking at other usages I found that core_message is used both ways in core. All usages needs to be made consistent and properly documented to avoid issues like this for themers.