Imports might not have any script after the last link import, in which case nothing will stop the import parent from firing DOMContentLoaded event before the that link import is fully loaded. (Imports have their own loadgroup, but those are contained by the import master, and not by the import parent).
Created attachment 8515924 [details] [diff] [review] Imports should block DOMContentLoaded of the parent. v1
sorry had to back this out for test failures like https://treeherder.mozilla.org/ui/logviewer.html#?job_id=3614488&repo=mozilla-inbound
Uhh... so we have some tests, testing features that we don't have yet and supposed to fail, and now they are failing differently than before... https://treeherder.mozilla.org/ui/#/jobs?repo=try&revision=b2765469f15c