sized_box_for_whitespace   
            
            SizedBox for whitespace.
This rule is available as of Dart 2.9.0.
Rule sets: flutter
This rule has a quick fix available.
Details
Use SizedBox to add whitespace to a layout.
A Container is a heavier Widget than a SizedBox, and as bonus, SizedBox
has a const constructor.
BAD:
Widget buildRow() {
  return Row(
    children: <Widget>[
      const MyLogo(),
      Container(width: 4),
      const Expanded(
        child: Text('...'),
      ),
    ],
  );
}
GOOD:
Widget buildRow() {
  return Row(
    children: const <Widget>[
      MyLogo(),
      SizedBox(width: 4),
      Expanded(
        child: Text('...'),
      ),
    ],
  );
}
Usage
To enable the sized_box_for_whitespace rule,
add sized_box_for_whitespace under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - sized_box_for_whitespace