I just ran the tests on the iOS test target and learned that this and all the other `NSMapTable` functions aren't available on iOS (yet). Additionally, the `NSMapTable` class itself was only added in iOS 6.

The easiest thing for now would be to make `NSMapTable` support OS X only. What do you think? If you agree, then let's make this class a target member of only the OS X framework and test targets, and make the `NSMapTable` tests conditional on:

```objc
#if TARGET_OS_MAC && !TARGET_OS_IPHONE
```