old_contents: 'use strict';
  
  // refer to https://github.com/zloirock/core-js
  import 'core-js/modules/es6.object.assign';
  import 'core-js/modules/es6.set';
  
new_contents: 'use strict';
  
  // refer to https://github.com/zloirock/core-js
  import 'core-js/modules/es6.object.assign';
  
subject: remove polyfill set