The stack property on the exception object should be read-write in JSC (is currently read-only) That it is read-only caused a layout test failure in the newly added fast/dom/DOMException/stack-trace.html as discussed in https://bugs.webkit.org/show_bug.cgi?id=86442
<rdar://problem/11458323>
Sounds like a duplicate of bug 86128.
*** This bug has been marked as a duplicate of bug 86128 ***