Add missing copyright on test cases.

pkg/front_end/testcases/argument.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.
+
 abstract class Base {}
 
 class Foo extends Base {}
 
 class Bar extends Base {}
 
 class Baz extends Base {}
 
 void foo(x) {}
 
 void bar(x) {}
 
 void foo_escaped(x) {}
 
 void bar_escaped(x) {}
 
 void escape(fn) {
   fn(new Baz());
 }
 
 main() {
   foo(new Foo());
   bar(new Bar());
   escape(foo_escaped);
   escape(bar_escaped);
 }