Thread safety + implicit import Dispatch

Author: johnno1962

Foundation/NSObject.swift

@@ -8,6 +8,7 @@
 //
 
 import CoreFoundation
+@_exported import Dispatch
 
 /// The `NSObjectProtocol` groups methods that are fundamental to all Foundation objects.
 ///

Foundation/URLSession/URLSession.swift

@@ -198,6 +198,7 @@ open class URLSession : NSObject {
     /// - Note: It's a **concurrent** queue.
     internal let taskAttributesIsolation: DispatchQueue 
     internal let taskRegistry = URLSession._TaskRegistry()
+    fileprivate let identiferLock = NSLock()
     fileprivate let identifier: Int32
     fileprivate var invalidated = false
     fileprivate static let registerProtocols: () = {
@@ -405,14 +406,12 @@ extension URLSession._Request {
 }
 
 fileprivate extension URLSession {
-    static let identiferLock = NSLock()
-
     func createNextTaskIdentifier() -> Int {
-        URLSession.identiferLock.lock()
-        let i = nextTaskIdentifier
-        nextTaskIdentifier += 1
-        URLSession.identiferLock.unlock()
-        return i
+        return identiferLock.synchronized {
+            let i = nextTaskIdentifier
+            nextTaskIdentifier += 1
+            return i
+        }
     }
 }