From the Swift documentation:
"If you define an optional variable without providing a default value,
the variable is automatically set to nil for you."
This simplifies the bridging story for Notifications to their objc counterparts since the id -> Any and AnyHashable changes have now been applied (which makes the previous boxing strategy no longer needed). Previous consumers of Notification that were using String keys should still work, however any explicit dictionary types should migrate from Swift 2.2 -> Swift 3 from userInfo as [NSObject:AnyObject] to [AnyHashable:Any]. The condition of distributed notifications (in non sandboxed apps) requiring plist types still applies and will fail at runtime if incorrect types are passed into the objective-c layer, and in the case of sandboxed apps userInfo still is forbidden (this change is a non functional change in the respect to those behaviors).
Resolves the following issues:
<rdar://problem/27426757>
<rdar://problem/27561621>
<rdar://problem/27259984>
All generic bridgeable types can bridge for all their instantiations now. Removing this ferrets out some now-unnecessary traps that check for unbridgeable parameter types.
Foundation provides a number of specific operators defined in the
global scope. Push all of these into their corresponding types. This
cleanup helps verify that the SE-0091 implementation is generally
functional.
