Add missing copyright on test cases.

pkg/front_end/testcases/optional.dart

+// 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";
   }
 }
 
 abstract class External {
   String externalMethod(int x, [int y, int z]);
   void listen(Listener listener);
 }
@@ skipping 29 matching lines @@
 
   extern.listen(new TestListener());
   extern.listen(new ExtendedListener());
   extern.listen(new InvalidListener());
 
   var nothing1 = foo.method();
   var nothing2 = foo.method(1, 2, 3, 4);
   var nothing3 = extern.externalMethod();
   var nothing4 = extern.externalMethod(1, 2, 3, 4);
 }