Since @wesbos answer didn't help, here is what I found out.
Sublime 3 does not retrieve packages. You will find your packages (on linux) in /opt/sublime_text/Packages/ for the default packages or ~/.config/sublime-text-3/Installed Packages for the packages you installed in the zip archive with the extension .sublime-package .
To change the contents of a package, install the package resource viewer and run the command : Open resource to go to the file you want to change ( if.sublime-snippet , I would also change for-()-{}.sublime-snippet , so as it has the same weird semicolon there) and edit it.
After saving the file, it will save it to ~/.config/sublime-text-3/Packages/JavaScript/if.sublime-snippet . This file then overwrites the default file in the original zip package.
As I understand it, it is important to know that the files that you overwrite in this way will not be updated when the packages are updated, since they overwrite everything that is in the updated package!
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