Apply suggestions from code review

Co-authored-by: Sébastien Doeraene <[email protected]>

Author: odersky

library/src/scala/util/boundary.scala

@@ -18,14 +18,14 @@ object boundary:
   /** User code should call `break.apply` instead of throwing this exception
    *  directly.
    */
-  class Break[T] private[boundary](val label: Label[T], val value: T)
+  final class Break[T] private[boundary](val label: Label[T], val value: T)
   extends RuntimeException(
     /*message*/ null, /*cause*/ null, /*enableSuppression=*/ false, /*writableStackTrace*/ false)
 
   /** Labels are targets indicating which boundary will be exited by a `break`.
    */
   final class Label[-T]:
-    def break(value: T): Nothing = throw Break(this, value)
+    private[util] def break(value: T): Nothing = throw Break(this, value)
 
   /** Run `body` with freshly generated label as implicit argument. Catch any
    *  breaks associated with that label and return their results instead of