[JS IR BE] Add Unit return type to function signature

libraries/stdlib/js-ir/runtime/long.kt

@@ -253,6 +253,7 @@ public class Long internal constructor(
     public override fun toDouble(): Double = toNumber()
 
     // This method is used by `toString()`
+    @JsName("valueOf")
     internal fun valueOf() = toDouble()
 
     override fun equals(other: Any?): Boolean = other is Long && equalsLong(other)

libraries/stdlib/js/src/kotlin/collections/AbstractMutableCollection.kt

@@ -44,6 +44,7 @@ public actual abstract class AbstractMutableCollection<E> protected actual const
         }
     }
 
+    @JsName("toJSON")
     open fun toJSON(): Any = this.toArray()
 }