I was considering whether to add an isExecutable() method to ArcanistLintEngine, but decided against it for no particular reason. It could be reasonable to add this method though.

Oh, one other thing I noticed is that Filesystem::changePermissions doesn't do any validation on $mode, which seems wrong to me given that PHP will do unexpected things if you pass a string to $mode (see http://php.net/manual/en/function.chmod.php#96086, for example).