LGTM. What error do you get when running tests locally?

@shyiko Would you be so kind as to review this as well?

I'd change error message to something simpler (e.g. Error on line: %d col %d\n%s -> %d:%d: %s) (or throw something like AssertionWithLineColumnError so that spotless could decide how to best display/hide location info based on cli arguments/configuration) but it's up to you guys.

@nedtwigg when running locally i get
java.lang.IllegalArgumentException: No such resource kotlin/licenseheader/KotlinCodeWithoutHeader.test

seems like the resources needed to run the test aren't being picked up from dependencies? i see testlib as a test dependency though.

Spotless chains a bunch of Function<String, String> together. If the first step adds or removes a bunch of header lines, then the second step throws an error, the line numbers in the error are going to be off. Because of this, the spotless project doesn't have an opinion for how line numbers should be formatted, and I don't think we should add API for an exceptions with built-in line numbers. Spotless is focused on the kinds of formatting errors that the formatter can fix itself, because those can be composed.

Feel free to format the line numbers as-is or according to @shyiko's advice, doesn't matter which way to us. I just did ./gradlew check for master on a mac and win 10 box, both succeeded. Not sure what might be wrong on your box, @runningcode, maybe try a clean?

Ok, I'll format it as-is. We can improve on it later when the need arises.

I guess my IDE can't find those resources files, but the tests work fine from the command line. Updated the tests!

Can you add a link to this PR into the changelog? That helps people review what happened, and helps them see how to make similar changes in the future.