Thread safety + implicit import Dispatch

Author: johnno1962

Foundation/NSObject.swift

@@ -13,6 +13,7 @@
 @_exported import Dispatch
 
 import CoreFoundation
+@_exported import Dispatch
 
 /// The `NSObjectProtocol` groups methods that are fundamental to all Foundation objects.
 ///

Foundation/URLSession/URLSession.swift

@@ -190,6 +190,7 @@ open class URLSession : NSObject {
     fileprivate var nextTaskIdentifier = 1
     internal let workQueue: DispatchQueue 
     internal let taskRegistry = URLSession._TaskRegistry()
+    fileprivate let identiferLock = NSLock()
     fileprivate let identifier: Int32
     fileprivate var invalidated = false
     fileprivate static let registerProtocols: () = {
@@ -395,8 +396,6 @@ extension URLSession._Request {
 }
 
 fileprivate extension URLSession {
-    static let identiferLock = NSLock()
-
     func createNextTaskIdentifier() -> Int {
         return workQueue.sync {
             let i = nextTaskIdentifier