Using the Dart pre-processor on several sites for a while now, it randomly
fails.
The result is that the cached css-file is blank, and the site is broken.
Quick-fix is clearing the cache.

The real fix for me is changing to the ScssPHP preprocessor.

I have not manages to reproduce the error, so it is unknown how and when it
happens.
But I guess that it happens because the command somehow fails, and the
$file_output is empty.
Then the cached css file is empty, and breaks the site untill a clear cache.

There should ba a sort of check, to see if there is a problem with the output
from the command line.
And if there is, it should fail. And no css file should be cached.
Not quite sure how this would work.
But if the input file is not empty, the output file should be populated with
something.

But now this preprocessor is a bit un-safe to use, cause it suddenly breaks a
site without notice.