JS backend: fixed wrong result when compare null and undefined using Kotlin.equals.

js/js.translator/testFiles/expression/equals/cases/equalsNullOrUndefined.kt

@@ -1,11 +1,18 @@
 package foo
 
+native val undefined: Any = noImpl
+
 fun box() : String {
     val a: Int? = null
     val r = a == null
     if (!r || a != null)
       return "wrong result on simple nullable check"
 
+    //force using Kotlin.equals
+    val t = null
+    if (t != undefined)
+      return "wrong result when compare null and undefined using Kotlin.equals"
+
     var i = 0;
     fun foo(): Int? = ++i;
     if (foo() == null)

js/js.translator/testFiles/kotlin_lib.js

@@ -36,8 +36,8 @@ var kotlin = {set:function (receiver, key, value) {
 
 (function () {
     Kotlin.equals = function (obj1, obj2) {
-        if (obj1 === null || obj1 === undefined) {
-            return obj2 === null;
+        if (obj1 == null) {
+            return obj2 == null;
         }
 
         if (obj1 instanceof Array) {