use_decorated_box  
            
            Use DecoratedBox.
This rule is available as of Dart 2.16.0.
This rule has a quick fix available.
Details
DO use DecoratedBox when Container has only a Decoration.
A Container is a heavier Widget than a DecoratedBox, and as bonus,
DecoratedBox has a const constructor.
BAD:
Widget buildArea() {
  return Container(
    decoration: const BoxDecoration(
      color: Colors.blue,
      borderRadius: BorderRadius.all(
        Radius.circular(5),
      ),
    ),
    child: const Text('...'),
  );
}
GOOD:
Widget buildArea() {
  return const DecoratedBox(
    decoration: BoxDecoration(
      color: Colors.blue,
      borderRadius: BorderRadius.all(
        Radius.circular(5),
      ),
    ),
    child: Text('...'),
  );
}
Usage
To enable the use_decorated_box rule,
add use_decorated_box under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - use_decorated_box