I've discovered this growing in an uncut section of field - leaves that remind me of bay or laurel arranged alternate with 1/4-1/2 inch black berries. I didn't see it when it was flowering.
Looks like a buckthorn.
Seriously invasive, at least here in New England.
"Seriously invasive, at least here in New England"
If you're thinking of either of the two European buckthorn species invasive in the USA, it isn't one of those. Check out Carolina Buckthorn Frangula caroliniana, that's a better fit, though I'm far from certain it is a buckthorn at all, they usually have more clustered berries.
Location of the plant would help.
Southeast Ontario. This guy may have more berries earlier in the season - there were only a few sparse ones when I discovered it, though.
I was thinking Aronia.