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

This function is available as C\gen_firstx() in Facebook's www repository.

Returns the first element of the result of the given Awaitable, or throws if the Traversable is empty

namespace HH\Lib\C;

function firstx_async<T>(
    Awaitable<Traversable<T>> $awaitable,
): Awaitable<T>;

For non-Awaitable Traversables, see C\firstx.