Fixes #2335. Fix superinterfaces_of_extension_type_A05_t07.dart, add new test expecting an error (#2336)

Author: sgrekhov

LanguageFeatures/Extension-types/superinterfaces_of_extension_type_A05_t07.dart

@@ -43,7 +43,7 @@
 import '../../Utils/expect.dart';
 
 extension type V(int id) {
-  external int id2;
+  external final int id2;
 }
 
 extension type ET(int id) implements V {

LanguageFeatures/Extension-types/superinterfaces_of_extension_type_A05_t08.dart

@@ -0,0 +1,54 @@
+// Copyright (c) 2023, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/// @assertion Assume that DV is an extension type declaration named Name, and
+/// V1 occurs as one of the <type>s in the <interfaces> of DV. In this case we
+/// say that V1 is a superinterface of DV.
+/// ...
+/// Assume that DV is an extension type declaration named Name, and the type V1,
+/// declared by DV1, is a superinterface of DV (V1 could be an extension type or
+/// a non-extension type). Let m be the name of a member of V1. If DV also
+/// declares a member named m then the latter may be considered similar to a
+/// declaration that "overrides" the former. However, it should be noted that
+/// extension type method invocation is resolved statically, and hence there is
+/// no override relationship among the two in the traditional object-oriented
+/// sense (that is, it will never occur that the statically known declaration is
+/// the member of V1, and the member invoked at run time is the one in DV). A
+/// receiver with static type V1 will invoke the declaration in DV1, and a
+/// receiver with a static type which is a reference to DV (like Name or
+/// Name<...>) will invoke the one in DV.
+///
+/// Hence, we use a different word to describe the relationship between a member
+/// named m of a superinterface, and a member named m which is declared by the
+/// subinterface: We say that the latter redeclares the former.
+///
+/// In particular, if two different declarations of m are inherited from two
+/// superinterfaces then the subinterface can resolve the conflict by
+/// redeclaring m.
+///
+/// There is no notion of having a 'correct override relation' here. With
+/// extension types, any member signature can redeclare any other member
+/// signature with the same name, including the case where a method is
+/// redeclared by a getter, or vice versa.
+///
+/// @description Checks that it is a compile-time error if an extension type
+/// declaration `DV` declares a member that conflicts with an inherited one
+/// @author [email protected]
+
+// SharedOptions=--enable-experiment=inline-class
+
+extension type V(int id) {
+  external int id2;
+}
+
+extension type ET(int id) implements V {
+  int id2() => id;
+//    ^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(ET);
+}