This API is part of the Hack Standard Library, not HHVM itself.
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This function is available as Dict\gen_filter() in Facebook's www repository.

Returns a new dict containing only the values for which the given async predicate returns true

namespace HH\Lib\Dict;

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

For non-async predicates, see Dict\filter().