Create method to json (#6111)

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

@@ -12,6 +12,10 @@ class {{classname}} {
   static const {{classname}} {{{name}}} = const {{classname}}._internal({{{value}}});
     {{/enumVars}}
   {{/allowableValues}}
+  
+  {{dataType}} toJson (){
+    return this.value;
+  }
 
   static {{classname}} fromJson({{dataType}} value) {
     return new {{classname}}TypeTransformer().decode(value);