Feature request: make it easier to start using json_serializable

*@roughike commented on Feb 10, 2018, 11:00 AM UTC:*

If `json_serializable` is the officially recommended JSON library for Dart / Flutter, it would make sense to make it easier to get started with it.

## A live template for json_serializable models

To create a `json_serializable` model, some amount of boilerplate code is required from the user. A live template makes the process much more convenient.

It would look something like this:

[](https://user-images.githubusercontent.com/13744304/35940545-37dd2748-0c58-11e8-8f68-50e0f0cadb1a.gif)

## Quick fixes for running code generation utility and adding needed dependencies

Here's the flow from the users point of view:

* create a `json_serializable` model class with the live template
* the `User` class has a squiggly red line under the name, saying "Code not generated yet"
* user presses Alt-Enter and sees a quick fix option called "Generate code for json_serializable model"

By hitting enter, the following steps would be done:

* Check if the `json_serializable` package is declared in pubspec file. If not, add it automatically and refresh the dependencies.
* Run `flutter packages pub run build_runner build` to build the generated code.

*This issue was moved by [Hixie](https://github.com/Hixie) from flutter/flutter/issues/14623.*