use_colored_box

Use a 'ColoredBox' rather than a 'Container' with only a 'Color'.

The analyzer produces this diagnostic when a Container is created that only sets the color.

The following code produces this diagnostic because the only attribute of the container that is set is the color:

import 'package:flutter/material.dart';

Widget build() {
  return Container(
    color: Colors.red,
    child: const Text('hello'),
  );
}

Replace the Container with a ColoredBox:

import 'package:flutter/material.dart';

Widget build() {
  return ColoredBox(
    color: Colors.red,
    child: const Text('hello'),
  );
}