This is a bug in Drupal 8 core, currently being worked on in several different core issues, many of which have working patches.
If you need the functionality now, you have two options:
Try the patches from comment #22 (or more recent) on Issue #2923634 and the patches on Issue #2752443. Read the comment thread on #2923634 to get a full understanding of what's going on here.
Or use the well-vetted patch from comment #139 on Issue #2118743 which has been postponed in favor of option 1 above.
I'm using the patch from option 2 (#2118743) successfully. Please note that this does go against the standard core contributor recommendations. But the reason is because #2118743 has a single, well-vetted and working patch for something that is only enabled / used during development, and instead of needing to test out various combinations of two separate patches that are still under development. Once the better solution (#2923634 and #2752443) get fixed in core, #2118743 should be obviated and no longer required.