invalid_use_of_covariant_in_extension

Can't have modifier '{0}' in an extension.

The analyzer produces this diagnostic when a member declared inside an extension uses the keyword covariant in the declaration of a parameter. Extensions aren't classes and don't have subclasses, so the keyword serves no purpose.

The following code produces this diagnostic because i is marked as being covariant:

extension E on String {
  void a(covariant int i) {}
}

Remove the covariant keyword:

extension E on String {
  void a(int i) {}
}