message: fix: checkPreflight
diff: export function checkPreflight() {
  + let p = Promise.resolve()
  if (this.opts.preflight) {
  - return this.preflightFun();
  + p = p.then(() => this.preflightFun());
  }
  if (this.opts.cdnPreflight) {
  - return this.cdnPreflightFun();
  + p = p.then(() => this.cdnPreflightFun())
  }
  - return Promise.resolve()
  + return p
  }
  