JS polyfill loader

Features.es6

class Features {
  static ensure(...features) {
    let requires = features.filter(it => it.check()).map(it => it.polyfill);
    return new Promise((res, rej) => {
      require(requires, (polyfills) => {
        try {
          features.forEach((it, idx) => {
            it.load(polyfills[idx]);
          });
          res();
        } catch (e) {
          rej(e);
        }
      });
    });
  }
}

Features.Promise = {
  check: function () {
    return !!window.Promise;
  },
  load: function (bluebird) {
    window.Promise = bluebird;
  },
  polyfill: 'bluebird'
};

Features.fetch = {
  check: function() {
    return !!window.fetch;
  },
  load: function (fetch) {
    window.fetch = fetch;
  },
  polyfill: 'fetch'
};

Features.ensure(Features.Promise, Features.fetch).then(run);