Deprecate access to the isa pointer. This will become a hard error in future.

objc/runtime.h

@@ -76,6 +76,9 @@ typedef struct objc_object
 	 * Pointer to this object's class.  Accessing this directly is STRONGLY
 	 * discouraged.  You are recommended to use object_getClass() instead.
 	 */
+#ifndef __OBJC_RUNTIME_INTERNAL__
+	__attribute__((deprecated))
+#endif
 	Class isa;
 } *id;