mixin_super_class_constraint_non_interface

Only classes and mixins can be used as superclass constraints.

The analyzer produces this diagnostic when a type following the on keyword in a mixin declaration is neither a class nor a mixin.

The following code produces this diagnostic because F is neither a class nor a mixin:

typedef F = void Function();

mixin M on F {}

If the type was intended to be a class but was mistyped, then replace the name.

Otherwise, remove the type from the on clause.