diff --git a/modules/openapi-generator/src/main/resources/dart-dio/api.mustache b/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
index 44d9c067416..95dc66a46e1 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/api.mustache
@@ -11,10 +11,12 @@ import 'package:built_value/serializer.dart';
 {{/fullImports}}
 
 class {{classname}} {
-    final Dio _dio;
-    Serializers _serializers;
 
-    {{classname}}(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const {{classname}}(this._dio, this._serializers);
 
     {{#operation}}
     /// {{{summary}}}
diff --git a/modules/openapi-generator/src/main/resources/dart-dio/apilib.mustache b/modules/openapi-generator/src/main/resources/dart-dio/apilib.mustache
index 645a5fe7877..62f2bfc06bc 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/apilib.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/apilib.mustache
@@ -10,31 +10,37 @@ import 'package:{{pubName}}/auth/oauth.dart';
 {{#apiInfo}}{{#apis}}import 'package:{{pubName}}/api/{{classFilename}}.dart';
 {{/apis}}{{/apiInfo}}
 
-final _defaultInterceptors = [OAuthInterceptor(), BasicAuthInterceptor(), ApiKeyAuthInterceptor()];
+final _defaultInterceptors = [
+  OAuthInterceptor(),
+  BasicAuthInterceptor(),
+  ApiKeyAuthInterceptor(),
+];
 
 class {{clientName}} {
 
-    Dio dio;
-    Serializers serializers;
-    String basePath = '{{{basePath}}}';
+    static const String basePath = r'{{{basePath}}}';
 
-    {{clientName}}({this.dio, Serializers serializers, String basePathOverride, List<Interceptor> interceptors}) {
-        if (dio == null) {
-            BaseOptions options = new BaseOptions(
+    final Dio dio;
+
+    final Serializers serializers;
+
+    {{clientName}}({
+      Dio dio,
+      Serializers serializers,
+      String basePathOverride,
+      List<Interceptor> interceptors,
+    })  : this.serializers = serializers ?? standardSerializers,
+          this.dio = dio ??
+              Dio(BaseOptions(
                 baseUrl: basePathOverride ?? basePath,
                 connectTimeout: 5000,
                 receiveTimeout: 3000,
-            );
-            this.dio = new Dio(options);
-        }
-
-        if (interceptors == null) {
-            this.dio.interceptors.addAll(_defaultInterceptors);
-        } else {
-            this.dio.interceptors.addAll(interceptors);
-        }
-
-        this.serializers = serializers ?? standardSerializers;
+              )) {
+      if (interceptors == null) {
+        this.dio.interceptors.addAll(_defaultInterceptors);
+      } else {
+        this.dio.interceptors.addAll(interceptors);
+      }
     }
 
     void setOAuthToken(String name, String token) {
diff --git a/modules/openapi-generator/src/main/resources/dart-dio/local_date_serializer.mustache b/modules/openapi-generator/src/main/resources/dart-dio/local_date_serializer.mustache
index 31440035f4a..68cfd5e31c6 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/local_date_serializer.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/local_date_serializer.mustache
@@ -4,6 +4,9 @@ import 'package:built_value/serializer.dart';
 import 'package:time_machine/time_machine.dart';
 
 class OffsetDateSerializer implements PrimitiveSerializer<OffsetDate> {
+
+  const OffsetDateSerializer();
+
   @override
   Iterable<Type> get types => BuiltList<Type>([OffsetDate]);
 
@@ -25,6 +28,9 @@ class OffsetDateSerializer implements PrimitiveSerializer<OffsetDate> {
 }
 
 class OffsetDateTimeSerializer implements PrimitiveSerializer<OffsetDateTime> {
+
+  const OffsetDateTimeSerializer();
+
   @override
   Iterable<Type> get types => BuiltList<Type>([OffsetDateTime]);
 
diff --git a/modules/openapi-generator/src/main/resources/dart-dio/serializers.mustache b/modules/openapi-generator/src/main/resources/dart-dio/serializers.mustache
index ec6464fca00..4697d00c323 100644
--- a/modules/openapi-generator/src/main/resources/dart-dio/serializers.mustache
+++ b/modules/openapi-generator/src/main/resources/dart-dio/serializers.mustache
@@ -26,8 +26,8 @@ Serializers serializers = (_$serializers.toBuilder(){{#apiInfo}}{{#apis}}{{#seri
         () => MapBuilder<String, {{baseType}}>(),
 {{/isMap}}
       ){{/serializers}}{{/apis}}{{/apiInfo}}{{#timeMachine}}
-      ..add(OffsetDateSerializer())
-      ..add(OffsetDateTimeSerializer()){{/timeMachine}}
+      ..add(const OffsetDateSerializer())
+      ..add(const OffsetDateTimeSerializer()){{/timeMachine}}
       ..add(Iso8601DateTimeSerializer()))
     .build();
 
diff --git a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api.dart b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api.dart
index 2422d33a0f3..2bdb6d8f833 100644
--- a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api.dart
+++ b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api.dart
@@ -18,31 +18,37 @@ import 'package:openapi/api/store_api.dart';
 import 'package:openapi/api/user_api.dart';
 
 
-final _defaultInterceptors = [OAuthInterceptor(), BasicAuthInterceptor(), ApiKeyAuthInterceptor()];
+final _defaultInterceptors = [
+  OAuthInterceptor(),
+  BasicAuthInterceptor(),
+  ApiKeyAuthInterceptor(),
+];
 
 class Openapi {
 
-    Dio dio;
-    Serializers serializers;
-    String basePath = 'http://petstore.swagger.io/v2';
+    static const String basePath = r'http://petstore.swagger.io/v2';
 
-    Openapi({this.dio, Serializers serializers, String basePathOverride, List<Interceptor> interceptors}) {
-        if (dio == null) {
-            BaseOptions options = new BaseOptions(
+    final Dio dio;
+
+    final Serializers serializers;
+
+    Openapi({
+      Dio dio,
+      Serializers serializers,
+      String basePathOverride,
+      List<Interceptor> interceptors,
+    })  : this.serializers = serializers ?? standardSerializers,
+          this.dio = dio ??
+              Dio(BaseOptions(
                 baseUrl: basePathOverride ?? basePath,
                 connectTimeout: 5000,
                 receiveTimeout: 3000,
-            );
-            this.dio = new Dio(options);
-        }
-
-        if (interceptors == null) {
-            this.dio.interceptors.addAll(_defaultInterceptors);
-        } else {
-            this.dio.interceptors.addAll(interceptors);
-        }
-
-        this.serializers = serializers ?? standardSerializers;
+              )) {
+      if (interceptors == null) {
+        this.dio.interceptors.addAll(_defaultInterceptors);
+      } else {
+        this.dio.interceptors.addAll(interceptors);
+      }
     }
 
     void setOAuthToken(String name, String token) {
diff --git a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
index 842f537de9e..9b4f286f41e 100644
--- a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
+++ b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
@@ -17,10 +17,12 @@ import 'package:built_collection/built_collection.dart';
 import 'package:openapi/api_util.dart';
 
 class PetApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    PetApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const PetApi(this._dio, this._serializers);
 
     /// Add a new pet to the store
     ///
diff --git a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart
index 38b37fa2e0c..0dce4aeeb87 100644
--- a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart
+++ b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart
@@ -14,10 +14,12 @@ import 'package:openapi/model/order.dart';
 import 'package:built_collection/built_collection.dart';
 
 class StoreApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    StoreApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const StoreApi(this._dio, this._serializers);
 
     /// Delete purchase order by ID
     ///
diff --git a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart
index 7dab8b5726e..7357aace8d7 100644
--- a/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart
+++ b/samples/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart
@@ -14,10 +14,12 @@ import 'package:openapi/model/user.dart';
 import 'package:built_collection/built_collection.dart';
 
 class UserApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    UserApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const UserApi(this._dio, this._serializers);
 
     /// Create user
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api.dart
index 2422d33a0f3..2bdb6d8f833 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api.dart
@@ -18,31 +18,37 @@ import 'package:openapi/api/store_api.dart';
 import 'package:openapi/api/user_api.dart';
 
 
-final _defaultInterceptors = [OAuthInterceptor(), BasicAuthInterceptor(), ApiKeyAuthInterceptor()];
+final _defaultInterceptors = [
+  OAuthInterceptor(),
+  BasicAuthInterceptor(),
+  ApiKeyAuthInterceptor(),
+];
 
 class Openapi {
 
-    Dio dio;
-    Serializers serializers;
-    String basePath = 'http://petstore.swagger.io/v2';
+    static const String basePath = r'http://petstore.swagger.io/v2';
 
-    Openapi({this.dio, Serializers serializers, String basePathOverride, List<Interceptor> interceptors}) {
-        if (dio == null) {
-            BaseOptions options = new BaseOptions(
+    final Dio dio;
+
+    final Serializers serializers;
+
+    Openapi({
+      Dio dio,
+      Serializers serializers,
+      String basePathOverride,
+      List<Interceptor> interceptors,
+    })  : this.serializers = serializers ?? standardSerializers,
+          this.dio = dio ??
+              Dio(BaseOptions(
                 baseUrl: basePathOverride ?? basePath,
                 connectTimeout: 5000,
                 receiveTimeout: 3000,
-            );
-            this.dio = new Dio(options);
-        }
-
-        if (interceptors == null) {
-            this.dio.interceptors.addAll(_defaultInterceptors);
-        } else {
-            this.dio.interceptors.addAll(interceptors);
-        }
-
-        this.serializers = serializers ?? standardSerializers;
+              )) {
+      if (interceptors == null) {
+        this.dio.interceptors.addAll(_defaultInterceptors);
+      } else {
+        this.dio.interceptors.addAll(interceptors);
+      }
     }
 
     void setOAuthToken(String name, String token) {
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
index e49f7800703..dacefa336bf 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/pet_api.dart
@@ -17,10 +17,12 @@ import 'package:built_collection/built_collection.dart';
 import 'package:openapi/api_util.dart';
 
 class PetApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    PetApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const PetApi(this._dio, this._serializers);
 
     /// Add a new pet to the store
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart
index 5adaabf7126..0c9f666e1b7 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/store_api.dart
@@ -14,10 +14,12 @@ import 'package:openapi/model/order.dart';
 import 'package:built_collection/built_collection.dart';
 
 class StoreApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    StoreApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const StoreApi(this._dio, this._serializers);
 
     /// Delete purchase order by ID
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart
index 82b1658a1ea..ce8c20111f0 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib/lib/api/user_api.dart
@@ -14,10 +14,12 @@ import 'package:openapi/model/user.dart';
 import 'package:built_collection/built_collection.dart';
 
 class UserApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    UserApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const UserApi(this._dio, this._serializers);
 
     /// Create user
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api.dart
index 3e1dec02f81..378be30a74a 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api.dart
@@ -22,31 +22,37 @@ import 'package:openapi/api/store_api.dart';
 import 'package:openapi/api/user_api.dart';
 
 
-final _defaultInterceptors = [OAuthInterceptor(), BasicAuthInterceptor(), ApiKeyAuthInterceptor()];
+final _defaultInterceptors = [
+  OAuthInterceptor(),
+  BasicAuthInterceptor(),
+  ApiKeyAuthInterceptor(),
+];
 
 class Openapi {
 
-    Dio dio;
-    Serializers serializers;
-    String basePath = 'http://petstore.swagger.io:80/v2';
+    static const String basePath = r'http://petstore.swagger.io:80/v2';
 
-    Openapi({this.dio, Serializers serializers, String basePathOverride, List<Interceptor> interceptors}) {
-        if (dio == null) {
-            BaseOptions options = new BaseOptions(
+    final Dio dio;
+
+    final Serializers serializers;
+
+    Openapi({
+      Dio dio,
+      Serializers serializers,
+      String basePathOverride,
+      List<Interceptor> interceptors,
+    })  : this.serializers = serializers ?? standardSerializers,
+          this.dio = dio ??
+              Dio(BaseOptions(
                 baseUrl: basePathOverride ?? basePath,
                 connectTimeout: 5000,
                 receiveTimeout: 3000,
-            );
-            this.dio = new Dio(options);
-        }
-
-        if (interceptors == null) {
-            this.dio.interceptors.addAll(_defaultInterceptors);
-        } else {
-            this.dio.interceptors.addAll(interceptors);
-        }
-
-        this.serializers = serializers ?? standardSerializers;
+              )) {
+      if (interceptors == null) {
+        this.dio.interceptors.addAll(_defaultInterceptors);
+      } else {
+        this.dio.interceptors.addAll(interceptors);
+      }
     }
 
     void setOAuthToken(String name, String token) {
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/another_fake_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/another_fake_api.dart
index bb8b4df8f38..21bcd8d0475 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/another_fake_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/another_fake_api.dart
@@ -13,10 +13,12 @@ import 'package:built_value/serializer.dart';
 import 'package:openapi/model/model_client.dart';
 
 class AnotherFakeApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    AnotherFakeApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const AnotherFakeApi(this._dio, this._serializers);
 
     /// To test special tags
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/default_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/default_api.dart
index 08d265f15a7..7dabdff912d 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/default_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/default_api.dart
@@ -13,10 +13,12 @@ import 'package:built_value/serializer.dart';
 import 'package:openapi/model/inline_response_default.dart';
 
 class DefaultApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    DefaultApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const DefaultApi(this._dio, this._serializers);
 
     /// 
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart
index e47ce652fe5..022866c3722 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_api.dart
@@ -21,10 +21,12 @@ import 'package:built_collection/built_collection.dart';
 import 'package:openapi/api_util.dart';
 
 class FakeApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    FakeApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const FakeApi(this._dio, this._serializers);
 
     /// Health check endpoint
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_classname_tags123_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_classname_tags123_api.dart
index 4e98faa733c..10092d010b4 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_classname_tags123_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/fake_classname_tags123_api.dart
@@ -13,10 +13,12 @@ import 'package:built_value/serializer.dart';
 import 'package:openapi/model/model_client.dart';
 
 class FakeClassnameTags123Api {
-    final Dio _dio;
-    Serializers _serializers;
 
-    FakeClassnameTags123Api(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const FakeClassnameTags123Api(this._dio, this._serializers);
 
     /// To test class name in snake case
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart
index aa51abb7fde..d01f1b2a8ba 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/pet_api.dart
@@ -17,10 +17,12 @@ import 'package:built_collection/built_collection.dart';
 import 'package:openapi/api_util.dart';
 
 class PetApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    PetApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const PetApi(this._dio, this._serializers);
 
     /// Add a new pet to the store
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/store_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/store_api.dart
index 3f770836b33..fa30f5d3d7b 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/store_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/store_api.dart
@@ -14,10 +14,12 @@ import 'package:openapi/model/order.dart';
 import 'package:built_collection/built_collection.dart';
 
 class StoreApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    StoreApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const StoreApi(this._dio, this._serializers);
 
     /// Delete purchase order by ID
     ///
diff --git a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/user_api.dart b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/user_api.dart
index eebeb895671..6e4eb642e18 100644
--- a/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/user_api.dart
+++ b/samples/openapi3/client/petstore/dart-dio/petstore_client_lib_fake/lib/api/user_api.dart
@@ -14,10 +14,12 @@ import 'package:openapi/model/user.dart';
 import 'package:built_collection/built_collection.dart';
 
 class UserApi {
-    final Dio _dio;
-    Serializers _serializers;
 
-    UserApi(this._dio, this._serializers);
+    final Dio _dio;
+
+    final Serializers _serializers;
+
+    const UserApi(this._dio, this._serializers);
 
     /// Create user
     ///