fix typo

Change-Id: I3ced675679187f2d54d2a7b03532ccc733949365
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352160
Auto-Submit: Jake Macdonald <jakemac@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Jake Macdonald <jakemac@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

tests/language/augmentation_libraries/class_augmentation_test.dart

diff --git a/tests/language/augmentation_libraries/class_augmentation_test.dart b/tests/language/augmentation_libraries/class_augmentation_test.dart
index ee2ae9f..30bba3e 100644
--- a/tests/language/augmentation_libraries/class_augmentation_test.dart
+++ b/tests/language/augmentation_libraries/class_augmentation_test.dart
@@ -84,7 +84,7 @@
 }
 
 mixin M {
-  String get mixin => 'a';
+  String get mixinX => 'a';
 }
 
 abstract interface class I {