makeInput = () =>
    freezeToBeSuppressTrapping({ __proto__: mercurialProto })
Do not freeze `mercurialProto` in order to test that an object with
a non-frozen __proto__ is not passable. Once we have support for
non-trapping, we should generalize this test (or add a new one) where
`mercurialProto` is frozen but still trapping. This test would then
test that a valid TagRecord must be non-trapping.

TODO In order to run this test before we have explicit support for a
non-trapping integrity trait, we have to `freeze` here but not `harden`.
However, once we do have that support, and `passStyleOf` checks that
its argument is also non-trapping, we still need to avoid `harden`
because that would also harden `__proto__`. So we will need to
explicitly make this non-trapping, which we cannot yet express.
@see https://github.com/endojs/endo/blob/master/packages/ses/docs/preparing-for-stabilize.md