The issue with Google's Compiler Closure service is that it has changed since the version of the library used in Flarum. One of those changes is that I believe it's stopped responding to HTTP requests. There's also something about externs but I don't think it effects Flarum since it doesn't use the "ADVANCED" compilation mode.
Updating the s9e/text-formatter dependency fixes it but I think that changing the URL manually can work as a band-aid until the dependency is updated. It should be removed as soon as the dependency is updated though:
$ccs->url = 'https://closure-compiler.appspot.com/compile';
Also, Flarum has its own cache as I recall. If you don't clear the cache, the first series of minifications could take a minute while the second series would take seconds.
