Fasta accepts conflicting interface type arguments when contravariant

```dart
class I<T> {}

class A {}

class B extends A {}

class X implements I<B> {}

class Y extends X implements I<A> {}
```
Is invalid, since `Y` implements two different generic instantiations of `I`. But Fasta does not report an error. If `A` and `B` are swapped, an error is correctly reported.