Add missing copyright on test cases.

pkg/front_end/testcases/override.dart

Index: pkg/front_end/testcases/override.dart
diff --git a/pkg/front_end/testcases/override.dart b/pkg/front_end/testcases/override.dart
index fbecca9fd2602fd2df8b29db948c0202f2b930e3..03f9c9f042edd9a1ff918a7d53d72f67980fcc93 100644
--- a/pkg/front_end/testcases/override.dart
+++ b/pkg/front_end/testcases/override.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 {}
 
 class Bar extends Foo {}