Different pattern for _span so it doesn't need to be late

Instead of marking the field nullable, or late, add a constructor in the
abstract class and use it from the others.

It was already impossible to correctly subclass `YamlNode` from outside
the library, so the addition of a private constructor should not be
breaking.
1 file changed
tree: 934ff3499e098a1bac9d71a2f94005a07697183b
  1. pkgs/