Fix errors and indent in built-ins sources

core/builtins/native/kotlin/Nothing.kt

@@ -20,4 +20,4 @@ package kotlin
  * Nothing has no instances. You can use Nothing to represent "a value that never exists": for example,
  * if a function has the return type of Nothing, it means that it never returns (always throws an exception).
  */
-public class Nothing private () {}
+public class Nothing private constructor()