JS: don't optimize when over external enum

See KT-21468

js/js.tests/test/org/jetbrains/kotlin/js/test/semantics/BoxJsTestGenerated.java

@@ -3388,6 +3388,12 @@ public class BoxJsTestGenerated extends AbstractBoxJsTest {
                 doTest(fileName);
             }
 
+            @TestMetadata("externalEnumSubject.kt")
+            public void testExternalEnumSubject() throws Exception {
+                String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/expression/when/externalEnumSubject.kt");
+                doTest(fileName);
+            }
+
             @TestMetadata("forWithOneStmWhen.kt")
             public void testForWithOneStmWhen() throws Exception {
                 String fileName = KotlinTestUtils.navigationMetadata("js/js.translator/testData/box/expression/when/forWithOneStmWhen.kt");

js/js.translator/src/org/jetbrains/kotlin/js/translate/expression/WhenTranslator.kt

@@ -243,7 +243,7 @@ private constructor(private val whenExpression: KtWhenExpression, context: Trans
     private fun KotlinType.getEnumClass(): ClassDescriptor? {
         if (isMarkedNullable) return null
         val classDescriptor = (constructor.declarationDescriptor as? ClassDescriptor)
-        return if (classDescriptor?.kind == ClassKind.ENUM_CLASS) classDescriptor else null
+        return if (classDescriptor?.kind == ClassKind.ENUM_CLASS && !classDescriptor.isExternal) classDescriptor else null
     }
 
     private fun translateEntryExpression(

js/js.translator/testData/box/expression/when/externalEnumSubject.js

@@ -0,0 +1,4 @@
+var X = {
+    A: {},
+    B: {}
+};

js/js.translator/testData/box/expression/when/externalEnumSubject.kt

@@ -0,0 +1,16 @@
+// EXPECTED_REACHABLE_NODES: 1133
+external enum class X {
+    A, B
+}
+
+fun box(): String {
+    if (foo(X.A) != "!") return "fail1: ${foo(X.A)}"
+    if (foo(X.B) != "@") return "fail2: ${foo(X.B)}"
+
+    return "OK"
+}
+
+fun foo(x: X) = when (x) {
+    X.A -> "!"
+    X.B -> "@"
+}