Add missing method to StripeJS mock

Our Angular code calls this method for interacting with live form validations and messages. We don't really need to use in tests, it just needs to exist otherwise the specs fail.
class Element {
  mount(el) {
    if (typeof el === "string") {
      el = document.querySelector(el);
    }

    el.innerHTML = `
      <input id="stripe-cardnumber" name="cardnumber" placeholder="Card number" size="16" type="text">
      <input name="exp-date" placeholder="MM / YY" size="6" type="text">
      <input name="cvc" placeholder="CVC" size="3" type="text">
    `;
  }

  addEventListener(event) {
    return true;
  }
}

window.Stripe = () => {
  const fetchLastFour = () => {
    return document.getElementById("stripe-cardnumber").value.substr(-4, 4);
  };

  return {
    elements: () => {
      return {
        create: (type, options) => new Element()
      };
    },

    createToken: card => {
      return new Promise(resolve => {
        resolve({ token: { id: "tok_123", card: { last4: fetchLastFour() } } });
      });
    }
  };
};

class Element {
  mount(el) {
    if (typeof el === "string") {
      el = document.querySelector(el);
    }

    el.innerHTML = `
      <input id="stripe-cardnumber" name="cardnumber" placeholder="Card number" size="16" type="text">
      <input name="exp-date" placeholder="MM / YY" size="6" type="text">
      <input name="cvc" placeholder="CVC" size="3" type="text">
    `;
  }
}

window.Stripe = () => {
  const fetchLastFour = () => {
    return document.getElementById("stripe-cardnumber").value.substr(-4, 4);
  };

  return {
    elements: () => {
      return {
        create: (type, options) => new Element()
      };
    },

    createToken: card => {
      return new Promise(resolve => {
        resolve({ token: { id: "tok_123", card: { last4: fetchLastFour() } } });
      });
    }
  };
};
