Heimdall actually used Dark Magic to summon the Bifrost without the aid of the actual bridge and gate. There's a few references to this in the other Thor movies, and in Infinity War he actually says:
"Allfathers. Let the Dark Magic flow through me one last time."
I don't recall it ever being explained exactly how the Bifrost works, but I doubt it will be used to save Stark. I place all my bets on Captain Marvel saving him, it would be a natural way to introduce her into the story and solve Tony's conundrum.