message: Change a let back to const
diff: @@ -78,7 +78,7 @@ export function babelCompile(forceCompile: boolean): RequestHandler {
  shouldTransform(request: Request, response: Response) {
  // We must never compile the Custom Elements ES5 Adapter or other
  // polyfills/shims.
  - let userAgentHeader = request.get('user-agent');
  + const userAgentHeader = request.get('user-agent');
  return !isPolyfill.test(request.url) &&
  compileMimeTypes.includes(getContentType(response)) &&
  (forceCompile || browserNeedsCompilation(userAgentHeader));
  