Fix issue #23914: do not attempt to make an already immutable array immutable

tests/language/issue_23914_test.dart

Index: tests/language/issue_23914_test.dart
diff --git a/tests/language/regress_22666_test.dart b/tests/language/issue_23914_test.dart
similarity index 77%
copy from tests/language/regress_22666_test.dart
copy to tests/language/issue_23914_test.dart
index bd19d39a53a3cf2f94466b298256c0091e39be8f..072f912b74cff3b0bbde89667b3ba30ea84ac71d 100644
--- a/tests/language/regress_22666_test.dart
+++ b/tests/language/issue_23914_test.dart
@@ -4,6 +4,6 @@
 
 import 'dart:collection';
 
-class A extends Object with LinkedListEntry<A> { }
-
-main() => new A();
+main() {
+  var a = new List.unmodifiable(new LinkedList());
+}