MOTO E 1st Gen From Dalvik to ART : com.android.systemui stop and loop
Don't ask why, don't ask me how ... Android automatically reverted to Dalvik.

My best guess is : there is an idiotproof security somewhere that revert automatically after to much fails (at least 20) on booting under the new runtime.
Maybe installing the motorola drivers on my computer before repetivly booting in normal and safe mode while the USB cable was plugged had a positive effect.
Maybe the little bug felt that i painfully resolved to fetch the electronic axe (aka hard reset) and flew away ...

It seems that no data was lost.
It's a true relief !


  If you cross this topic, and face the same trouble, well ... it took me five day of patience, two full charge and discharge of battery,
  stubborn booting and prompting closing, ten cache wipe, and utter
  infuriation before everything settled down by itself. Try your luck !


Kaldui.

PS. To the android engineer : i was mistaken, you weren't making overtime to pay the attorney for your divorce, yo mama is a fine women, she ain't this overweight, and her reputation is of a loving, caring and faithful person. I also own you a beer.