buildTileHolders
Builds a holder for every entry in this list via buildTileHolder — the standard way a container tile's TileHolderBuilder turns its own schema's tiles field into the tiles constructor argument of the TileHolder it's building.
Receiver
the schema's own declared children, or null if it declares none.
Return
a mutable list of built holders, in the same order — empty (not null) when the receiver is null.