In the slow march towards weaning ourselves off YUI, i've just removed the mcore YUI rollup from being included on every single Moodle page (MDL-57690).
This was including a few Moodle YUI modules unconditionally all the time (dock loader, popup help, and widget/dialogue modules) and so if you are using of any of these modules without explicitly declaring the dependency it may not have been detected before. Now that those modules are not being included unconditionally your code may break.
It would be a good time to test your plugins early against master this week using boost. Or grep your code for instances of
M.core.dialogue (which was the main thing we detected undeclared dependencies in core code).