Convert some "mixed-in field" tests into unit tests

[srawlins]

These "mixed-in field" tests are a little complex, involving four types and a diamond-shaped inheritance graph. But they are meaningful; sort of the smallest diamond-shape of inheritance. I struggled to make sense of the end-to-end tests (as a change I was making was breaking them) and just had to convert them to unit tests to make them make sense 😅 .

[szakarias]

LGTM

[szakarias]

I am not sure how important this is, but are we maybe missing this test case in the new (and much nicer) setup?

If I understand it correctly we are testing that a field in a subclass identifies the correct enclosing container even though there is a mixin (that does not override the field)? Something like:

    class C {
      int x = 0;
    }
    class D extends C {
      @override
      int x = 1;
    }
    mixin M on C {
      // M does not override x
    }
    class E extends D with M {}
    ```

[srawlins]

Hmm, you're right... I replaced each test one-by-one, I should have covered everything, but yes it looks like I'm missing this case. I think I am getting a cold; maybe my brain is foggy 😅 .