ARKit, Xcode, and iOS 11 upgrade
In order to develop ARKit apps you need to use Xcode 9.x. Xcide 8.2.x does not include ARKit.

From Apple's Developer page:


  Get Started with ARKit
  
  Xcode 9 and iOS 11 Download iOS 11 and the latest version of Xcode 9,
  which includes the iOS 11 SDK to build AR features in your app today.