empty_container_bodies

Empty {0} bodies should be written using a ';' rather than '{}'.

The analyzer produces this diagnostic when the block body of a class, enum, extension, extension type, or mixin is empty and can be replaced by a semicolon.

The following code produces this diagnostic because the body of the class C is empty:

class C {}

If the class needs to have members, then add them:

class C {
  String toString() => 'c';
}

Otherwise, replace the body with a semicolon:

class C;