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

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

Returns a new dict formed by merging the KeyedTraversable elements of the given Traversable


namespace HH\Lib\Dict;

function flatten<Tk as arraykey, Tv>(
  Traversable<KeyedTraversable<Tk, Tv>> $traversables,
): dict<Tk, Tv>;

In the case of duplicate keys, later values will overwrite the previous ones.

For a fixed number of KeyedTraversables, see Dict\merge().