[stdlib] Explicit visibility and return types: builtins

core/builtins/src/kotlin/Unit.kt

@@ -20,5 +20,5 @@ package kotlin
  * The type with only one value: the `Unit` object. This type corresponds to the `void` type in Java.
  */
 public object Unit {
-    override fun toString() = "kotlin.Unit"
+    override fun toString(): String = "kotlin.Unit"
 }