code: function then(onFulfilled, onRejected) {
        var reaction = newPromiseCapability(speciesConstructor(this, $Promise));
        reaction.ok = typeof onFulfilled == 'function' ? onFulfilled : true;
        reaction.fail = typeof onRejected == 'function' && onRejected;
        reaction.domain = isNode ? process.domain : undefined;
        this._c.push(reaction);
        if (this._a) this._a.push(reaction);
        if (this._s) notify(this, false);
        return reaction.promise;
      }
docs: 25.4.5.3 Promise.prototype.then(onFulfilled, onRejected)