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

js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/BoxJsTestGenerated.java

@@ -3824,6 +3824,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
             public void testSimpleIsInterface() throws Exception {
                 runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsInterface.kt");
             }
+
+            @Test
+            @TestMetadata("simpleIsObject.kt")
+            public void testSimpleIsObject() throws Exception {
+                runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsObject.kt");
+            }
         }
 
         @Nested

js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/fir/FirJsBoxTestGenerated.java

@@ -4440,6 +4440,12 @@ public class FirJsBoxTestGenerated extends AbstractFirJsBoxTest {
             public void testSimpleIsInterface() throws Exception {
                 runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsInterface.kt");
             }
+
+            @Test
+            @TestMetadata("simpleIsObject.kt")
+            public void testSimpleIsObject() throws Exception {
+                runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsObject.kt");
+            }
         }
 
         @Nested

js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsES6TestGenerated.java

@@ -4546,6 +4546,12 @@ public class IrBoxJsES6TestGenerated extends AbstractIrBoxJsES6Test {
             public void testSimpleIsInterface() throws Exception {
                 runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsInterface.kt");
             }
+
+            @Test
+            @TestMetadata("simpleIsObject.kt")
+            public void testSimpleIsObject() throws Exception {
+                runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsObject.kt");
+            }
         }
 
         @Nested

js/js.tests/tests-gen/org/jetbrains/kotlin/js/test/ir/IrBoxJsTestGenerated.java

@@ -4440,6 +4440,12 @@ public class IrBoxJsTestGenerated extends AbstractIrBoxJsTest {
             public void testSimpleIsInterface() throws Exception {
                 runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsInterface.kt");
             }
+
+            @Test
+            @TestMetadata("simpleIsObject.kt")
+            public void testSimpleIsObject() throws Exception {
+                runTest("js/js.translator/testData/box/expression/typeCheck/simpleIsObject.kt");
+            }
         }
 
         @Nested

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"
+}