[FIR] Serialize generated classes into Klib metadata

^KT-57781 Fixed

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/kotlin/org/jetbrains/kotlin/gradle/K2KotlinxSerializationIT.kt

@@ -63,4 +63,12 @@ class K2KotlinxSerializationIT : KGPBaseTest() {
             build(":compileProductionExecutableKotlinJs")
         }
     }
+
+    @DisplayName("Compile test sourceset with kotlinx.serialization to JS. KT-57781")
+    @GradleTest
+    fun `test kotlinx serialization compile test source set to JS`() {
+        project("kotlinxSerializationK2WithJs", gradleVersion) {
+            build(":compileTestKotlinJs")
+        }
+    }
 }

libraries/tools/kotlin-gradle-plugin-integration-tests/src/test/resources/testProject/kotlinxSerializationK2WithJs/src/test/kotlin/SimpleTest.kt

@@ -0,0 +1,5 @@
+class SimpleTest {
+    fun test() {
+        ErrorResponse.serializer()
+    }
+}