Fix typedef cycle check (fixes #25620).

tests/language/regress_25620_test.dart

Index: tests/language/regress_25620_test.dart
diff --git a/tests/language/accessor_conflict_getter.dart b/tests/language/regress_25620_test.dart
similarity index 71%
copy from tests/language/accessor_conflict_getter.dart
copy to tests/language/regress_25620_test.dart
index 5b8d72fad773b7cf48fd0e0f9b40c3cd8669628d..7ea9a6f05c3d365b15c3e3b5a6eb78ea87d7d0fc 100644
--- a/tests/language/accessor_conflict_getter.dart
+++ b/tests/language/regress_25620_test.dart
@@ -2,6 +2,7 @@
 // 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.
 
-var getValue;
+import "dart:async";
+typedef Future<T> SyncedExecutionFn<T>(Future<T> fn());
+main() {}
 
-get x => getValue;