Failing _.delay test in webkit

I just ran the tests from current master in Gnome Web 50.6. All tests passed except for _.delay:

It seems to run the assertions as intended initially, but then there is this cryptic error about global variables (which I could not find in the source code) and apparently an unexpected third assertion is run. I have a hunch this is due to a WebKit bug or a Gnome Web bug, but it deserves some investigation.