Breaking Change: Support @JS at library level

tests/html/js_typed_interop_anonymous_unreachable_exp_test.dart

 // Copyright (c) 2015, 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.
 //
 // SharedOptions=--experimental-trust-js-interop-type-annotations
 
 // Same test as js_typed_interop_anonymous_unreachable, but using the
 // --experimental-trust-js-interop-type-annotations flag.
+@JS()
 library js_typed_interop_anonymous_unreachable_exp_test;
 
 import 'dart:html';
 import 'dart:js' as js;
 
 import 'package:js/js.dart';
 import 'package:unittest/unittest.dart';
 import 'package:unittest/html_config.dart';
 
 @JS() @anonymous
 class Literal {
   external factory Literal({int x, String y, num z});
 
   external int get x;
   external String get y;
   external num get z;
 }
 
 main() {
   useHtmlConfiguration();
   test('nothing to do', () {
     // This test is empty, but it is a regression for Issue# 24974: dartjs
     // would crash trying to compile code that used @anonymous and that was
     // not reachable from main.
   });
 }