JS: tests for KT-7357: extension functions from external KJS library couldn't be called in some cases

libraries/examples/browser-example-with-library/src/main/kotlin/sample/Hello.kt

@@ -3,6 +3,7 @@ package sample
 import kotlin.Pair
 import kotlin.browser.document
 import library.sample.*
+import kotlin.js.Date
 
 fun myApp() {
     val element = document.getElementById("foo")
@@ -11,6 +12,7 @@ fun myApp() {
         val x = pairAdd(p)
         val y = pairMul(p)
         val z = IntHolder(100).value
-        element.appendChild(document.createTextNode("x=$x y=$y z=$z")!!)
+        val u = Date().extFun()
+        element.appendChild(document.createTextNode("x=$x y=$y z=$z u=$u")!!)
     }
 }