It was supposed to be a surprise, but guessing from the hundreds of people massed outside Stubb’s at 7 p.m. on Thursday, it wasn’t exactly hush-hush. Already in town to promote their new fan-shot concert documentary, Awesome; I F—in’ Shot That!, the Beasties dropped by the outdoor venue for an early-evening run through some classics from their catalog, including “Paul Revere,” “Pass the Mic,” and “Brass Monkey.” It wasn’t all a nostalgia trip though: Mixmaster Mike kept it current by mashing in new-school beats — Snoop’s “Drop It Like It’s Hot,” for example — underneath the decidedly old-school rhymes.