Summary:
In the version of JSC on iOS 11, creating a JSContext on one
thread and using it on another can trigger subtle and nearly
impossible to debug reentrancy-related crashes in the VM (see
https://bugs.webkit.org/show_bug.cgi?id=186827). In !NDEBUG builds,
check for this case and throw an exception, so it can be detected
early.
Reviewed By: amnn
Differential Revision: D13313264
fbshipit-source-id: ee85435c20e23c8520495ce743d2f91f2eeada5c
bdb084e8a8