Add error for non-covariant type variable in superinterface (#3412)

Add compile-time error for non-covariant type variable in superinterface, just like the rule we have for classes, mixins, etc. It may be sound to allow this, but for now we prefer the simplicity and consistency of having the same rule everywhere. Also, the ability to violate this constraint does not seem to allow for anything useful.

accepted/future-releases/extension-types/feature-specification.md

@@ -16,6 +16,8 @@ information about the process, including in their change logs.
 [2]: https://github.com/dart-lang/language/blob/master/working/extension_structs/overview.md
 
 2023.10.18
+  - Add error for non-covariant occurrence of a type variable in a
+    superinterface.
   - Specify how to promote the representation variable of an extension
     type.
 
@@ -1142,6 +1144,16 @@ representation type of _DV_.
 non-extension type, it must be sound to consider the representation object
 as having type `T`.*
 
+A compile-time error occurs if a type variable declared by _DV_ occurs in a
+non-covariant position in `V1`.
+
+*This error corresponds to a similar error for class, mixin, mixin class,
+and enum declarations. It might be possible to relax the rule for extension
+types without jeopardizing soundness, but at this time we prefer the
+simplicity and consistency of having the same rule for all kinds of
+declarations. Also, it seems unlikely that a relaxation of the rule would
+open the door for anything particularly useful.*
+
 A compile-time error occurs if any direct or indirect superinterface of
 _DV_ denotes the declaration _DV_. *As usual, subtype cycles are not
 allowed.*