Explanatory comment

Author: johnno1962

Foundation/NSObject.swift

@@ -7,9 +7,13 @@
 // See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 //
 
-import CoreFoundation
+// @_exported import of Dispatch here makes it available to all
+// classes in Foundation and all sources that import Foundation.
+// This brings it into line with Darwin usage for compatbility.
 @_exported import Dispatch
 
+import CoreFoundation
+
 /// The `NSObjectProtocol` groups methods that are fundamental to all Foundation objects.
 ///
 /// If an object conforms to this protocol, it can be considered a first-class object.