code: function supportsMapAndSet() {
    if (typeof Symbol === 'undefined' || typeof Map === 'undefined' || typeof Set === 'undefined') {
      return false;
    }
    var prop = Object.getOwnPropertyDescriptor(Map, Symbol.species);
    return prop && 'get' in prop && Map[Symbol.species] === Map;
  }
docs: /* global Map,Set,Symbol Based on https://kangax.github.io/compat-table/es6/ we can sniff out incomplete Map/Set implementations which would otherwise cause our tests to fail. Notably they fail in IE11 and iOS 8.4, which this prevents.