Add test for unimplemented IsolateType

test/isolate_manager_shared_test.dart

@@ -4,6 +4,7 @@ import 'package:isolate_manager/isolate_manager.dart';
 import 'package:test/test.dart';
 
 import '../test/isolate_manager_test.dart';
+import 'models/user.dart';
 
 /*
   dart run isolate_manager:generate -i test -o test --shared --worker-mappings-experiment=test/isolate_manager_shared_test.dart
@@ -204,6 +205,11 @@ void main() async {
       await isolates.stop();
     });
 
+    test('Unimplemented Type', () {
+      final user = User(name: 'user', email: 'user@user.com');
+      expect(() => IsolateType.encode(user), throwsUnimplementedError);
+    });
+
     test('num', () async {
       final value = IsolateNum(15);
 

test/models/user.dart

@@ -0,0 +1,30 @@
+import 'dart:convert';
+
+class User {
+  User({
+    required this.name,
+    required this.email,
+  });
+
+  factory User.fromMap(Map<String, dynamic> map) {
+    return User(
+      name: map['name'] as String,
+      email: map['email'] as String,
+    );
+  }
+
+  factory User.fromJson(String source) =>
+      User.fromMap(json.decode(source) as Map<String, dynamic>);
+
+  final String name;
+  final String email;
+
+  Map<String, dynamic> toMap() {
+    return {
+      'name': name,
+      'email': email,
+    };
+  }
+
+  String toJson() => json.encode(toMap());
+}