[K/JS] Add ability to use is checks on external objects

js/js.translator/testData/box/expression/typeCheck/simpleIsObject.kt

@@ -0,0 +1,22 @@
+// EXPECTED_REACHABLE_NODES: 1403
+// IGNORE_BACKEND: JS
+
+package foo
+
+object RegularKotlinObject
+
+@JsName("Array")
+external object JsArray
+
+@JsName("Math")
+external object JsMath
+
+fun box(): String {
+    if (js("[]") !is JsArray) return "fail1"
+    if (js("[]") is JsMath) return "fail2"
+    if (js("[]") is RegularKotlinObject) return "fail3"
+    if (JsMath !is JsMath) return "fail4"
+    if (RegularKotlinObject !is RegularKotlinObject) return "fail5"
+
+    return "OK"
+}