Central as beta:
https://treeherder.mozilla.org/#/jobs?repo=try&resultStatus=testfailed%2Cbusted%2Cexception%2Cusercancel%2Crunnable&revision=99dac234054854bcb3eee882bf692630957d6cc9&selectedJob=286530272

Failure log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=286530272&repo=try&lineNumber=12994

Snippet:
[task 2020-01-26T12:05:52.850Z] 12:05:52 INFO - REFTEST TEST-START | js/src/tests/non262/expressions/object-literal-accessor-arguments.js
[task 2020-01-26T12:05:52.850Z] 12:05:52 INFO - REFTEST TEST-LOAD | file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/jsreftest.html?test=non262/expressions/object-literal-accessor-arguments.js | 1289 / 12177 (10%)
[task 2020-01-26T12:05:52.866Z] 12:05:52 INFO - TEST-INFO | 536472: ES5: { get x(v) { } } and { set x(v, v2) { } } should be syntax errors
[task 2020-01-26T12:05:52.902Z] 12:05:52 INFO - REFTEST TEST-PASS | js/src/tests/non262/expressions/object-literal-accessor-arguments.js | item 1
[task 2020-01-26T12:05:52.903Z] 12:05:52 INFO - REFTEST TEST-END | js/src/tests/non262/expressions/object-literal-accessor-arguments.js
[task 2020-01-26T12:05:52.919Z] 12:05:52 INFO - REFTEST TEST-START | js/src/tests/non262/expressions/object-literal-accessor-property-name.js
[task 2020-01-26T12:05:52.920Z] 12:05:52 INFO - REFTEST TEST-LOAD | file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/jsreftest.html?test=non262/expressions/object-literal-accessor-property-name.js | 1290 / 12177 (10%)
[task 2020-01-26T12:05:52.941Z] 12:05:52 INFO - TEST-INFO | 715682: Permit numbers and strings containing numbers as accessor property names
[task 2020-01-26T12:05:52.978Z] 12:05:52 INFO - REFTEST TEST-PASS | js/src/tests/non262/expressions/object-literal-accessor-property-name.js | item 1
[task 2020-01-26T12:05:52.978Z] 12:05:52 INFO - REFTEST TEST-END | js/src/tests/non262/expressions/object-literal-accessor-property-name.js
[task 2020-01-26T12:05:52.993Z] 12:05:52 INFO - REFTEST TEST-START | js/src/tests/non262/expressions/object-literal-computed-property-evaluation.js
[task 2020-01-26T12:05:52.994Z] 12:05:52 INFO - REFTEST TEST-LOAD | file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/jsreftest.html?test=non262/expressions/object-literal-computed-property-evaluation.js | 1291 / 12177 (10%)
[task 2020-01-26T12:05:53.015Z] 12:05:53 INFO - TEST-INFO | 1199546: Convert computed property name expressions to property key before evaluating the property's value
[task 2020-01-26T12:05:53.016Z] 12:05:53 INFO - TEST-INFO | Tests complete
[task 2020-01-26T12:05:53.061Z] 12:05:53 INFO - REFTEST TEST-PASS | js/src/tests/non262/expressions/object-literal-computed-property-evaluation.js | item 1
[task 2020-01-26T12:05:53.062Z] 12:05:53 INFO - REFTEST TEST-END | js/src/tests/non262/expressions/object-literal-computed-property-evaluation.js
[task 2020-01-26T12:05:53.078Z] 12:05:53 INFO - REFTEST TEST-START | js/src/tests/non262/expressions/octal-literals.js
[task 2020-01-26T12:05:53.079Z] 12:05:53 INFO - REFTEST TEST-LOAD | file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/jsreftest.html?test=non262/expressions/octal-literals.js | 1292 / 12177 (10%)
[task 2020-01-26T12:05:53.096Z] 12:05:53 INFO - TEST-INFO | 894026: Implement ES6 octal literals
[task 2020-01-26T12:05:53.096Z] 12:05:53 INFO - TEST-INFO | Tests complete
[task 2020-01-26T12:05:53.135Z] 12:05:53 INFO - REFTEST TEST-PASS | js/src/tests/non262/expressions/octal-literals.js | item 1
[task 2020-01-26T12:05:53.135Z] 12:05:53 INFO - REFTEST TEST-END | js/src/tests/non262/expressions/octal-literals.js
[task 2020-01-26T12:05:53.150Z] 12:05:53 INFO - REFTEST TEST-START | js/src/tests/non262/expressions/optional-chain.js
[task 2020-01-26T12:05:53.151Z] 12:05:53 INFO - REFTEST TEST-LOAD | file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/jsreftest.html?test=non262/expressions/optional-chain.js | 1293 / 12177 (10%)
[task 2020-01-26T12:05:53.173Z] 12:05:53 INFO - TEST-INFO | 1566143: Implement the Optional Chain operator (?.) proposal
[task 2020-01-26T12:05:53.173Z] 12:05:53 INFO - TEST-INFO | FAILED! Error: TypeError has wrong message!, expected can't access property "b", (intermediate value).a is undefined but got (intermediate value).a is undefined
[task 2020-01-26T12:05:53.173Z] 12:05:53 INFO - JavaScript error: file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/non262/expressions/optional-chain.js, line 49: Error: TypeError has wrong message!, expected can't access property "b", (intermediate value).a is undefined but got (intermediate value).a is undefined
[task 2020-01-26T12:05:53.193Z] 12:05:53 INFO - REFTEST TEST-UNEXPECTED-FAIL | js/src/tests/non262/expressions/optional-chain.js | Unknown file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/non262/expressions/optional-chain.js:49: Error: TypeError has wrong message!, expected can't access property "b", (intermediate value).a is undefined but got (intermediate value).a is undefined item 1
[task 2020-01-26T12:05:53.194Z] 12:05:53 INFO - REFTEST INFO | Saved log: START file:///builds/worker/workspace/build/tests/jsreftest/tests/js/src/tests/jsreftest.html?test=non262/expressions/optional-chain.js
[task 2020-01-26T12:05:53.195Z] 12:05:53 INFO - REFTEST INFO | Saved log: [CONTENT] OnDocumentLoad triggering AfterOnLoadScripts
[task 2020-01-26T12:05:53.195Z] 12:05:53 INFO - REFTEST INFO | Saved log: Initializing canvas snapshot
[task 2020-01-26T12:05:53.196Z] 12:05:53 INFO - REFTEST INFO | Saved log: [CONTENT] AfterOnLoadScripts belatedly entering WaitForTestEnd

Test added in bug 1566143, jsreftests didn't run for previous simulations. Information how to create beta simulations can be found at https://wiki.mozilla.org/Sheriffing/How_To/Beta_simulations If a patch gets attached to this bug, sheriffs will add it to their next beta simulation to verify that it works.

The new error message was added in bug #1259822. However we should be able to rely on the core part of the error message, as it shouldn't change too much due to webcompat issues. I changed the test so that it accepts more forms of the same message. There is no change to functionality.

This looks like it is ready to land, aryx -- do i need to do anything before landing it?

Thank you for fixing it quickly. Just land it, if there are issues, code sheriffs would reopen this bug.

https://hg.mozilla.org/mozilla-central/rev/0efc92de3d13

Verified fixed in latest beta sims: https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=9d3e16d3e0b94a907ad816513685f8f7d4d63ec7&searchStr=j1&selectedJob=286815382

*** I added the jobs manually to see if they turn green, they did not run by default in the above push.