Set correct type for enum values list (fixes #28341).

tests/language/regress_28341_test.dart

+// Copyright (c) 2017, 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.
+
+import "package:expect/expect.dart";
+
+enum E {
+  A
+}
+
+main() {
+  Expect.isTrue(E.values == const <E>[E.A]);
+  Expect.isFalse(E.values == const [E.A]);
+}