I think bug 1485664 broke the ability to evaluate expressions in workers. The debugger uses the console's evaluateJSAsync request for this. Before these patches, each worker was associated with its own console actor which could evaluate expressions in that worker's thread, but after the patch in https://phabricator.services.mozilla.com/D20150 each worker is associated with the same console actor which the main thread uses.
Unfortunately there was no test for this functionality (mea culpa, windowless workers are pretty new and the tests are in poor shape). I've attached a patch which updates a mochitest to exercise this. This test passes before these patches, and fails afterwards.
I'm not sure how to go about fixing this. Is there still a separate console actor associated with each thread? From the changes in D20150 it seems like there isn't, but it's hard for me to follow the changes that are happening in this bug.