newSwap doubleSwap intermittent test failure on XS

[dckc]

While working on #3078 I saw a test failure in ci run 2558992363 for b646e7d (based on master= 25d5d43 ) that I don't think is closely related:

  message: 'newSwap doubleSwap threw: cannot coerce object to string',
  filename: 'test/unitTests/contracts/newSwap/test-newSwap-swap.js',

1 tests failed
F test/unitTests/contracts/newSwap/test-newSwap-swap.js newSwap doubleSwap

reproduced with

• yarn test:xs-unit test/unitTests/contracts/newSwap/test-newSwap-swap.js

but failed to reproduce with

• yarn test:xs-unit test/unitTests/contracts/newSwap/test-newSwap-swap.js -m 'newSwap doubleSwap'
• yarn test:xs-unit --debug test/unitTests/contracts/newSwap/test-newSwap-swap.js

That it doesn't reproduce with -m is only a mild inconvenience. But that it goes away with XS --debug makes it look pretty expensive to diagnose.

[dckc]

I'm inclined to postpone this until some xsnap debug instrumentation (#2578) is in place.

[dckc]

@phoddie points out that cannot coerce object to string happens whenever we do:

const o = { __proto__: null };
const s = `${o}`;

p.s. I checked... node throws in this case too; so how is it in our code?

p.p.s escalated as endojs/endo#729 .

[dckc]

I think this was fixed in endojs/endo#772 , which was merged Jun 9. Agoric SDK uses 0.13.2 (2021-06-14) according to 1876df0