WASHINGTON - There's a thrilling sequence in the movie The Dark Knight in which Batman upends an oncoming tractor trailer driven by the Joker. The truck flips over vertically, a crash that ought to have left no survivors, yet somehow the Joker pulls himself out of the wreckage, throws his shoulders back, and stands in the middle of street, daring the bigger, stronger superhero to come after him again. He shouts, "COME ON! HIT ME!" And Batman revs up the Batcycle and speeds straight at him, only to veer away at the last instant, leaving his nemesis standing there, triumphant, ready to continue their confrontation.