JSON conversion in Flutter and Dart

The article Parsing Complex JSON in Flutter is very good, but I was looking for something shorter, a quick reference with examples.

If I have a simple JSON string like this:

{
    "name" : "Mary",
    "age" : 30
}

How do I convert it to a Dart object? Or a Dart object to JSON?

Setup

Import the builtin convert library, which includes the jsonDecode() and jsonEncode() methods:

import 'dart:convert';