Fixes #2117. Update assertions, add tests for external members

Language/Expressions/Method_Invocation/Super_Invocation/accessible_instance_member_t01.dart

@@ -10,8 +10,8 @@
 /// Ssuper, and let F be the return type of D. If both lookups failed, a
 /// compile-time error occurs
 ///
-/// @description Checks that it is a compile error if `S` does not have
-/// an instance member named `m`.
+/// @description Checks that it is a compile error if loockup of an instance
+/// member named `m` in `S` failed.
 /// @author msyabro
 
 class S {}

Language/Expressions/Method_Invocation/Super_Invocation/accessible_instance_member_t02.dart

@@ -10,8 +10,8 @@
 /// Ssuper, and let F be the return type of D. If both lookups failed, a
 /// compile-time error occurs
 ///
-/// @description Checks that it is a compile error if member `m` in `S` is
-/// inaccessible.
+/// @description Checks that it is a compile error if loockup of an instance
+/// member named `m` in `S` failed because member `m` in `S` is inaccessible.
 /// @author msyabro
 
 import '../lib.dart';

Language/Expressions/Method_Invocation/Super_Invocation/accessible_instance_member_t03.dart

@@ -10,8 +10,8 @@
 /// Ssuper, and let F be the return type of D. If both lookups failed, a
 /// compile-time error occurs
 ///
-/// @description Checks that it is a compile-time error if a superclass does not
-/// have an instance member named `m`
+/// @description Checks that it is a compile-time error if loockup for an
+/// instance member named `m` failed
 /// @author [email protected]
 /// @issue 52901
 

Language/Expressions/Method_Invocation/Super_Invocation/accessible_instance_member_t04.dart

@@ -0,0 +1,34 @@
+// 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 Let Ssuper be the superclass of the immediately enclosing class
+/// for i, and let L be the library that contains i. Let the declaration D be
+/// the result of looking up the method m in Ssuper with respect to L, and let F
+/// be the static type of D. Otherwise, if the method lookup failed, let the
+/// declaration D be the result of looking up the getter m with respect to L in
+/// Ssuper, and let F be the return type of D. If both lookups failed, a
+/// compile-time error occurs
+///
+/// @description Checks that it is not an error if loockup for an instance
+/// member named `m` successes. Test an external member (with no body but not an
+/// abstract)
+/// @author [email protected]
+
+abstract class A {
+  external void m();
+  external int g;
+  external void set s(int i);
+}
+
+class C extends A {
+  void n() {
+    super.m();
+    super.g;
+    super.s = 0;
+  }
+}
+
+main() {
+  print(C);
+}