JS: use prototype when checking is against interface

See KT-20527

js/js.translator/testData/box/rtti/prototypeUsedToFindInterface.kt

@@ -0,0 +1,25 @@
+// EXPECTED_REACHABLE_NODES: 1112
+interface A {
+    fun foo(): String
+}
+
+class B : A {
+    override fun foo(): String = "OK"
+}
+
+fun box(): String {
+    val b = B::class.js
+    val c = js("""
+    function C() {
+        b.call(this);
+    };
+    C.prototype = Object.create(b.prototype);
+    C.prototype.constructor = C;
+    new C();
+    """)
+
+    if (c !is B) return "fail: c !is B"
+    if (c !is A) return "fail: c !is A"
+
+    return "OK"
+}