This constructor was creating an "invalid" `PropertyKey`, but every
code path constructing such a `PropertyKey` was either never used or
immediately `VERIFY`ing that the `PropertyKey` was valid.
The `VERIFY` call has been moved to the `PropertyKey::from_value(...)`
call, and the array/object literal spreading code could be refactored
to not take a `PropertyKey` but creates dummy values for now.
The default constructor for `Reference` has similairly be deleted,
because it was never used.