checks for overrides need fixing

[sigmundch]

We seem to be incorrectly checking overrides in some scenarios, for example mixins and transitive checks.

class A {}
class B extends A {}

class Base {
  A f1;
}
class M {
  B f1;
}

class Test1 extends Base {
  B f1;
}
class Test2 extends Test1 {
  B f1;
}
class Test3 extends M implements Base {}
class Test4 extends Object with M implements Base {}
class Test5 extends Base with M {}

This currently produces 2 errors, on Test1 and Test2. The one in Test2 is redundant - should we report it?

Test3 through 5 report no errors today, but they should.

[sigmundch]

Also - how do we report errors for Test3..Test5 - techincally the error is in the application of the mixin, not in the mixin declaration itself