ses 0.12.5 vs ava-xs tests: Cannot assign to read only property 'hasOwnProperty'

[dckc]

Describe the bug

Since the SES 0.12.6 upgrade (#2718), the ERTP and zoe tests run with ava-xs fail:

> @agoric/ertp
$ yarn test:xs-unit && yarn test:xs-worker
$ ava-xs
...
(TypeError#1)
TypeError#1: Cannot assign to read only property 'hasOwnProperty' of object '[object Object]'

  at Generator.next (<anonymous>)
  at runMicrotasks (<anonymous>)

-- https://github.com/Agoric/agoric-sdk/runs/2204161291#step:7:90

recent example: https://github.com/Agoric/agoric-sdk/runs/2220491959#step:7:98

To Reproduce

1. in packages/ERTP, run yarn test:xs-unit
2. see error as above