[K/JS] Prepare JS Plain Objects plugin to publication

plugins/js-plain-objects/compiler-plugin/testData/box/optional.kt

@@ -0,0 +1,31 @@
+package foo
+
+import kotlinx.js.JsPlainObject
+
+@JsPlainObject
+external interface User {
+    var name: String
+    val age: Int
+    val email: String?
+}
+
+fun box(): String {
+    val user = User(name = "Name", age = 10)
+
+    if (user.name != "Name") return "Fail: problem with `name` property"
+    if (user.age != 10) return "Fail: problem with `age` property"
+
+    val json = js("JSON.stringify(user)")
+    if (json != "{\"email\":null,\"age\":10,\"name\":\"Name\"}") return "Fail: got the next json: $json"
+
+    val withEmail = User(name = "Name", age = 10, email = "test@test")
+
+    if (withEmail.name != "Name") return "Fail: problem with emailed `name` property"
+    if (withEmail.age != 10) return "Fail: problem with emailed `age` property"
+    if (withEmail.email != "test@test") return "Fail: problem with emailed `email` property"
+
+    val jsonWithEmail = js("JSON.stringify(withEmail)")
+    if (jsonWithEmail != "{\"email\":\"test@test\",\"age\":10,\"name\":\"Name\"}") return "Fail: got the next emailed json: $jsonWithEmail"
+
+    return "OK"
+}