fix(file-list): do not preprocess up-to-date files

[segrey]

It improves performance of "karma run" command which refreshes fileList.

Closes #2829

[segrey]

@johnjbarton What do you think about this change? I can add tests if the change is generally OK.

[johnjbarton]

Should we worry that we will perform the check even though the preprocess maybe trivial?
Is every preprocess known to be cached?

[segrey]

Should we worry that we will perform the check even though the preprocess maybe trivial?

As I understand, the check is fast enough and shouldn't impact the performance. Also, if I'm not mistaken, fileList.refresh() is called from karma run command only (lib/middleware/runner.js). AFAIK, karma run is not used often when running karma tests manually.

Is every preprocess known to be cached?

Looks like. At least fileList.changeFile(filePath) runs preprocessors for changed files only. It seems OK to preprocess changed files only in fileList.refresh() too.

[johnjbarton]

Ok please see if you can create a test.

[segrey]

Done, please take a look.

[Havunen]

This PR probably broke single entry setups? #3226