We skip serializing any of the internal state of the Regex<ECMA262>
object, because that state is all computable from the input pattern
and flags. If it turns out that this is really, really slow, we can add
some optimizations to serialize more of the regex parse result.
The invariants for these property getters are supposed to be checked by
the has_entries_and_events_disabled AO, but we don't have all the
plumbing hooked up between Navigables and Navigation yet.
Add a test to make sure that these methods don't assert when calling
them on a fresh page.
