There are several places where we want to be able to read the contents of a directory which may be either bundled in a JAR, or unpacked in the file system. Right now, we have separate logic in all of these places, some better than others.

And, in particular, the easiest way to check for the existence of a file in these cases is to just try making a request for it (either via fetch/XHR, or by creating a nsIChannel). This is also just about the most expensive way to perform such a check.

We should have a standard helper that makes this easier to do correctly than incorrectly.