[JS IR BE] Make kotlinx.io compile

- Fix expect/actual default arguments
 - Fix dynamic type in inliner
 - Fix external varargs

js/js.translator/testData/box/jsModule/externalClass.kt

@@ -7,12 +7,24 @@ external class A(x: Int = definedExternally) {
     val x: Int
 
     fun foo(y: Int): Int = definedExternally
+
+    fun bar(vararg arg: String): String = definedExternally
 }
 
+class C {
+    val e = arrayOf("e")
+    val f = arrayOf("f")
+    val a = A(1)
+
+    fun qux() = a.bar(*e, *f)
+}
+
+
 fun box(): String {
     val a = A(23)
     assertEquals(23, a.x)
     assertEquals(65, a.foo(42))
+    assertEquals(C().qux(), "(ef)")
 
     return "OK"
 }