The fix to <https://bugs.webkit.org/show_bug.cgi?id=173321> - "DFG doesn't properly handle a property that is change to read only in a prototype" is to add a check for ReadOnly to the AbsenceOfSetter in PropertyCondition::isStillValidAssumingImpurePropertyWatchpoint(). Talking to Phil Pizlo it makes sense to reflect the changed functionality in the name of AbsenceOfSetter in all of it's uses.
Settled on AbsenceOfSetEffects.
Created attachment 312816 [details]
Comment on attachment 312816 [details]
Committed r218218: <http://trac.webkit.org/changeset/218218>