Add missing copyright on test cases.

pkg/front_end/testcases/closure.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 {
   var _field = new Bar();
 }
 
 class Bar {}
 
 useCallback(callback) {
   var _ = callback();
 }
 
 main() {
   var x;
   inner() {
     x = new Foo();
     return new Foo();
   }
 
   useCallback(inner);
   var _ = inner()._field;
 }