#2559. Add augmenting enum values test. (#2921)

LanguageFeatures/Augmentation-libraries/augmenting_enum_values_A03_t04.dart

@@ -0,0 +1,38 @@
+// Copyright (c) 2024, 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 New enum values may be defined in an augmenting enum, and they
+/// will be appended to the current values of the declaration in augmentation
+/// application order.
+///
+/// @description Checks that it is a syntax error if an enum augmentation
+/// contains no any values.
+/// @author [email protected]
+
+// SharedOptions=--enable-experiment=macros
+
+enum E1 {
+  e0;
+}
+
+augment enum E1 {}
+//           ^^
+// [analyzer] unspecified
+// [cfe] unspecified
+
+enum E2 {
+  e0;
+}
+
+augment enum E2 {
+  void foo() {}
+//^^^^
+// [analyzer] unspecified
+// [cfe] unspecified
+}
+
+main() {
+  print(E1);
+  print(E2);
+}