node_modules/core-js/library/modules/es6.weak-set.js

'use strict';
var weak = require('./_collection-weak');
var validate = require('./_validate-collection');
var WEAK_SET = 'WeakSet';

// 23.4 WeakSet Objects
require('./_collection')(WEAK_SET, function (get) {
  return function WeakSet() { return get(this, arguments.length > 0 ? arguments[0] : undefined); };
}, {
  // 23.4.3.1 WeakSet.prototype.add(value)
  add: function add(value) {
    return weak.def(validate(this, WEAK_SET), value, true);
  }
}, weak, false, true);