[JS IR BE] Use Char boxing rules of current backend

js/js.translator/testData/box/coercion/charValParameter.kt

@@ -1,14 +1,15 @@
-// IGNORE_BACKEND: JS_IR
-// EXPECTED_REACHABLE_NODES: 1283
+// EXPECTED_REACHABLE_NODES: 1276
 class A(val x: Char)
 
 fun typeOf(x: dynamic): String = js("typeof x")
 
+val expectedCharRepresentationInProperty = if (testUtils.isLegacyBackend()) "object" else "number"
+
 fun box(): String {
     val a = A('0')
 
     var r = typeOf(a.asDynamic().x)
-    if (r != "object") return "fail1: $r"
+    if (r != expectedCharRepresentationInProperty) return "fail1: $r"
 
     r = typeOf(a.x)
     if (r != "number") return "fail2: $r"