getExpiration() {
    // This will equal Infinity if there are no entries.
    let min = Infinity;

    const sessions = this.activeSessions_.keys();
    for (const session of sessions) {
      if (!isNaN(session.expiration)) {
        min = Math.min(min, session.expiration);
      }
    }

    return min;
  }
Returns the next expiration time, or Infinity.
@return {number}