Add missing copyright on test cases.

pkg/front_end/testcases/closure.dart

Index: pkg/front_end/testcases/closure.dart
diff --git a/pkg/front_end/testcases/closure.dart b/pkg/front_end/testcases/closure.dart
index ccb50b06c77008bf527192ba052d6cc8bffb9971..664a4fa0897d2694ab1948c3171067134e44b43e 100644
--- a/pkg/front_end/testcases/closure.dart
+++ b/pkg/front_end/testcases/closure.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 {
   var _field = new Bar();
 }