Dart - generate constructor with named params (#6751)

* Dart - generate constructor with names params

* Test if constructor exists

modules/openapi-generator/src/main/resources/dart2/class.mustache

@@ -6,7 +6,12 @@ class {{classname}} {
   {{#min}} // range from {{min}} to {{max}}{{/min}}//{{^min}}enum {{name}}Enum { {{#values}} {{.}}, {{/values}} };{{/min}}{
   {{/allowableValues}}
   {{/vars}}
-  {{classname}}();
+
+  {{classname}}({
+  {{#vars}}
+    this.{{name}},
+  {{/vars}}
+  });
 
   @override
   String toString() {

samples/client/petstore/dart2/petstore/test/pet_test.dart

@@ -25,11 +25,12 @@ void main() {
         ..id = 124321
         ..name = 'Jose'
     ];
-    return Pet()
-      ..id = id
-      ..category = category
-      ..tags = tags
-      ..name = name
+    return Pet(
+      id : id,
+      category: category,
+      tags: tags,
+      name: name,
+    )
       ..status = status
       ..photoUrls = ['https://petstore.com/sample/photo1.jpg'];
   }

samples/client/petstore/dart2/petstore_client_lib/lib/model/api_response.dart

@@ -7,7 +7,12 @@ class ApiResponse {
   String type = null;
   
   String message = null;
-  ApiResponse();
+
+  ApiResponse({
+    this.code,
+    this.type,
+    this.message,
+  });
 
   @override
   String toString() {

samples/client/petstore/dart2/petstore_client_lib/lib/model/category.dart

@@ -5,7 +5,11 @@ class Category {
   int id = null;
   
   String name = null;
-  Category();
+
+  Category({
+    this.id,
+    this.name,
+  });
 
   @override
   String toString() {

samples/client/petstore/dart2/petstore_client_lib/lib/model/order.dart

@@ -14,7 +14,15 @@ class Order {
   //enum statusEnum {  placed,  approved,  delivered,  };{
   
   bool complete = false;
-  Order();
+
+  Order({
+    this.id,
+    this.petId,
+    this.quantity,
+    this.shipDate,
+    this.status,
+    this.complete,
+  });
 
   @override
   String toString() {

samples/client/petstore/dart2/petstore_client_lib/lib/model/pet.dart

@@ -14,7 +14,15 @@ class Pet {
   /* pet status in the store */
   String status = null;
   //enum statusEnum {  available,  pending,  sold,  };{
-  Pet();
+
+  Pet({
+    this.id,
+    this.category,
+    this.name,
+    this.photoUrls,
+    this.tags,
+    this.status,
+  });
 
   @override
   String toString() {

samples/client/petstore/dart2/petstore_client_lib/lib/model/tag.dart

@@ -5,7 +5,11 @@ class Tag {
   int id = null;
   
   String name = null;
-  Tag();
+
+  Tag({
+    this.id,
+    this.name,
+  });
 
   @override
   String toString() {

samples/client/petstore/dart2/petstore_client_lib/lib/model/user.dart

@@ -17,7 +17,17 @@ class User {
   String phone = null;
   /* User Status */
   int userStatus = null;
-  User();
+
+  User({
+    this.id,
+    this.username,
+    this.firstName,
+    this.lastName,
+    this.email,
+    this.password,
+    this.phone,
+    this.userStatus,
+  });
 
   @override
   String toString() {