Trying out Swift 4 in Xcode 9 Beta.

For some reason I'm getting a crash when using key value accessors on an NSObject.

Any ideas?

```
import Cocoa

class Person: NSObject {
    var name = ""
    var age = 0
}

let alpha = Person()
alpha.name = "Robert"
alpha.age = 53

alpha.value(forKey: "name")
// error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).

```