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

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

Like filter_async, but lets you utilize the keys of your dict too

namespace HH\Lib\Dict;

function filter_with_key_async<Tk as arraykey, Tv>(
    KeyedContainer<Tk, Tv> $traversable,
  (function(Tk,Tv):Awaitable<bool>) $predicate,
): Awaitable<dict<Tk, Tv>>;

For non-async filters with key, see Dict\filter_with_key().