Object.terminateAllThreads crash during shutdown

[kripken]

e.g.

0923/235942.356948:INFO:CONSOLE(2184)] "Uncaught abort(Assertion failed: undefined) at Error
    at jsStackTrace (http://localhost:8888/test.js:1927:17)
    at stackTrace (http://localhost:8888/test.js:1944:16)
    at abort (http://localhost:8888/test.js:1687:44)
    at assert (http://localhost:8888/test.js:741:5)
    at Object.terminateAllThreads (http://localhost:8888/test.js:2088:11)
    at exit (http://localhost:8888/test.js:5119:13)
    at Worker.worker.onmessage (http://localhost:8888/test.js:2181:19)", source: http://localhost:8888/test.js (2184)

This happens during shutdown, and the test has already reported success or failure, so this doesn't cause breakage on CI.

It looks like an assertion there assumes all threads have been cleaned up properly, while terminateAllThreads does it all at once, so the assumption is wrong. But I'm not sure.