Add example allOf with single ref (#10948)

* Add example allOf with single ref

* fix dart-dio-next handling of that case

* Refactor without vendor extension

* Regenerate newer samples

samples/client/petstore/javascript-promise-es6/test/model/UserType.spec.js

@@ -0,0 +1,58 @@
+/**
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ *
+ */
+
+(function(root, factory) {
+  if (typeof define === 'function' && define.amd) {
+    // AMD.
+    define(['expect.js', process.cwd()+'/src/index'], factory);
+  } else if (typeof module === 'object' && module.exports) {
+    // CommonJS-like environments that support module.exports, like Node.
+    factory(require('expect.js'), require(process.cwd()+'/src/index'));
+  } else {
+    // Browser globals (root is window)
+    factory(root.expect, root.OpenApiPetstore);
+  }
+}(this, function(expect, OpenApiPetstore) {
+  'use strict';
+
+  var instance;
+
+  beforeEach(function() {
+  });
+
+  var getProperty = function(object, getter, property) {
+    // Use getter method if present; otherwise, get the property directly.
+    if (typeof object[getter] === 'function')
+      return object[getter]();
+    else
+      return object[property];
+  }
+
+  var setProperty = function(object, setter, property, value) {
+    // Use setter method if present; otherwise, set the property directly.
+    if (typeof object[setter] === 'function')
+      object[setter](value);
+    else
+      object[property] = value;
+  }
+
+  describe('UserType', function() {
+    it('should create an instance of UserType', function() {
+      // uncomment below and update the code to test UserType
+      //var instance = new OpenApiPetstore.UserType();
+      //expect(instance).to.be.a(OpenApiPetstore.UserType);
+    });
+
+  });
+
+}));