I’ve just release version 1.0.0-beta.1 of the Elm Language Support package for Sublime Text, with experimental support for Elm 0.19.
If you’ve upgraded to Elm 0.19, you should install this pre-release version and give me feedback on how it works for you!
If you rely on Elm 0.18 support, you should configure Package Control to ignore new releases of the package, since the next regular release is likely to drop Elm 0.18 support.
Either way, everything you need to know is in the README.