Add missing copyright on test cases.

pkg/front_end/testcases/argument.dart

Index: pkg/front_end/testcases/argument.dart
diff --git a/pkg/front_end/testcases/argument.dart b/pkg/front_end/testcases/argument.dart
index db3906362b571ac4d69e2197be60321a239e8c9b..8642ce919248aa33d3e6bfc98481e88a38b2e479 100644
--- a/pkg/front_end/testcases/argument.dart
+++ b/pkg/front_end/testcases/argument.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.
+
 abstract class Base {}
 
 class Foo extends Base {}