JsonToDart

Considering that it uses overall less data and hence, can increase the parsing speed, there is no surprise that JSON is generally preferred over HTTP. Then again, JSON comes with its own limitations especially when working with large databases and response objects from API calls, as it becomes increasingly challenging to parse.
As the name suggests, JsonToDart is a tool designed to help you convert JSON data to Dart or Flutter, where the data is better structured in nested classes and therefore, makes the processing of API calls simpler.

Allows you to input a file or text directly

Following a quick and uneventful installation, you are welcomed by a clean and fresh single-window interface that is split into two main sections, one for inputting the code and another where you can preview and edit JSON keys. As suggested in the left panel, you can simple paste the JSON code or add a file to get started.
In the right panel is where you can make all necessary modifications, which should be fairly more convenient now that the class structures are clearly displayed. The first column shows the keys in JSON, the second one is for the class name and third is where you can find the property name. According to the developer, the app highlights any missing information with a red background, whereas a yellow background indicates that there is a class.

A simple and straightforward tool for converting JSON into class objects

The tool includes a few settings that you may find useful, including Data Protection as well as date and time, and thus enables you to add your copyright to the newly generated code. To add data protection, add your markers in the dedicated field from Settings.
In the eventuality that you need to manage JSON code and handle API calls and class objects better, but are having troubles converting the code to DART, then perhaps, you can consider giving JsonToDart a try.