This was failing to take two things into account:
- When constructing a PropertyName from a value, it won't automatically
convert to Type::Number for something like string "0", even though
that's how things work internally, since indexed properties are stored
separately. This will be improved in a future patch, it's a footgun
and should happen automatically.
- Those can't be looked up on the shape, we have to go through the
indexed properties instead.
Additionally it now operates on the shape or indexed properties directly
as define_property() was overly strict and would throw if a property was
already non-configurable.
Fixes#6469.
