Even though calling delete on a super property will ultimately throw a
ReferenceError, we must return the allocated register for the result of
the delete operation (which would normally be a boolean). If the delete
operation is used in a return statement, the bytecode generator for the
return statement must be able to assume the statement had some output.
- We have to check if the property name is a string before calling
as_string() on it
- We can't as_number() the same property name but have to use the parsed
index number
Fixes#3950.
