buckthorn vs chokecherry
That is Common Buckthorn (Rhamnus cathartica).
If one looks closely, one can distinguish the plant characters to tell Buckthorn from Chokecherry (Prunus virginiana). For starters, the buds look different. On Buckthorn, they are arranged subopposite and are appressed, meaning they are pressed closed to the stem; there's also a tiny thorn at the end of the stem (ie. Buckthorn). Chokecherry buds are alternate and distinctly pointy. The shape of the leaves is different, too. On Buckthorn, the leaves are rounder; on Chokecherry the leaves are longer and pointier and there are axillary tufts of hair and glands on the petiole.
One clear difference in your photos is the fruit: Buckthorn flower - which is inconspicuous - and fruit is in umbels while Chokecherry flowers - which are very showy and fragrant - hang in long, racemes.