KT-40235 null companion for stdlib external union types

js/js.translator/testData/box/regression/companionObjectInExternalInterface.kt

@@ -3,6 +3,7 @@
 // This hack is used in org.w3c.* part of standard library to represent unions of Strings
 // Test that we are not actually trying to access nonexistent companion object
 
+@JsName("null")
 @Suppress("NESTED_CLASS_IN_EXTERNAL_INTERFACE")
 public external interface I {
     companion object
@@ -14,4 +15,4 @@ fun box(): String {
     if (I.O != I.O) return "Fail 1"
     if (I.O == I.K) return "Fail 2"
     return I.O.unsafeCast<String>() + I.K.unsafeCast<String>()
-}
+}