This API is part of the Hack Standard Library, not HHVM itself.
Facebook Engineer?

This function is available as Dict\gen_map() in Facebook's www repository.

Returns a new dict where each value is the result of calling the given async function on the original value

namespace HH\Lib\Dict;

function map_async<Tk as arraykey, Tv1, Tv2>(
    KeyedTraversable<Tk, Tv1> $traversable,
  (function(Tv1):Awaitable<Tv2>) $value_func,
): Awaitable<dict<Tk, Tv2>>;

For non-async functions, see Dict\map().