Add missing copyright on test cases.

pkg/front_end/testcases/bad_store.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;
 }
 
 dynamic identity(x) => x;
 
 void use(x) {}
 
 main(List<String> args) {
   dynamic foo = identity(new Foo());
   if (args.length > 1) {
     foo.field = "string";
     var first = foo.field;
     use(first);
     foo.noField = "string";
     var second = foo.noField;
     use(second);
   }
 }