Add missing copyright on test cases.

pkg/front_end/testcases/prefer_baseclass.dart

Index: pkg/front_end/testcases/prefer_baseclass.dart
diff --git a/pkg/front_end/testcases/prefer_baseclass.dart b/pkg/front_end/testcases/prefer_baseclass.dart
index 40061c3425f9e15efc41611b88cfe7baadb8dc35..394322b245481fb3c53ac7c81af51ae6bce0fcf5 100644
--- a/pkg/front_end/testcases/prefer_baseclass.dart
+++ b/pkg/front_end/testcases/prefer_baseclass.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 A {}
 
 class B {}