[analyzer] Fix setter in test

Change-Id: Ie5d31dcf300936690d9cf476bed64b8615474e8d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/265090
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/test/src/utilities/extensions/library_element_test.dart

@@ -171,7 +171,7 @@ mixin ^M {}
 
   void test_setter() async {
     await assertLocation('''
-Set f^(String v) => '';
+set f^(String v) {}
 ''');
   }