Add missing copyright on test cases.

pkg/front_end/testcases/optional.dart

Index: pkg/front_end/testcases/optional.dart
diff --git a/pkg/front_end/testcases/optional.dart b/pkg/front_end/testcases/optional.dart
index becc9eadc3ce7e95d6a32ce95a4bf9f1bdf98ad7..9a278ee3657f1cb67c94cfa4d93729e32c6ca469 100644
--- a/pkg/front_end/testcases/optional.dart
+++ b/pkg/front_end/testcases/optional.dart
@@ -1,3 +1,7 @@
+// 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.
+
 class Foo {
   method(x, [y, z]) {
     return "string";