EXT:core has hidden dependency on EXT:frontend (circular dependency)
In ext_tables.php of EXT:core, classes from EXT:frontend are used. However, EXT:frontend is not declared as dependency of EXT:core (since this would create a circular dependency).
The problem is usage of class constants from a class that does not exist without EXT:frontend installed.
Suggest moving these constants to EXT:core (since they relate to "pages" table which is supposed to be operable even without EXT:frontend installed and is fully declared as SQL in EXT:core).
IMHO this would still need to be fixed/backported to v9. There's another case that was fixed recently in EXT:extbase - https://github.com/TYPO3/TYPO3.CMS/commit/49dcd95090faffb7931131348c6d9ac17428998f
Until this is fixed in v9 the circular dependency still exists therein.