[BUG] Dart mustache model toJson doesn't handle the situation which the property is object or array

Created by: winterbesos

https://github.com/OpenAPITools/openapi-generator/blob/96bbab98a47c702c9b05f7f2bf06f485071dbd23/modules/openapi-generator/src/main/resources/dart2/class.mustache#L86

If there is a property is an object or an array, it dose not handle this situation.