use_colored_box  
            
            Use ColoredBox.
This rule is available as of Dart 2.17.0.
Details
DO use ColoredBox when Container has only a Color.
A Container is a heavier Widget than a ColoredBox, and as bonus,
ColoredBox has a const constructor.
BAD:
Widget buildArea() {
  return Container(
    color: Colors.blue,
    child: const Text('hello'),
  );
}
GOOD:
Widget buildArea() {
  return const ColoredBox(
    color: Colors.blue,
    child: Text('hello'),
  );
}
Usage
To enable the use_colored_box rule,
add use_colored_box under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - use_colored_box