The BookBrainz codebase will be using a lot more ES6 features in the future, since they make code more readable and easier to maintain.

Node 5.x natively supports these features, so please upgrade rika and sakura to use the latest Node version. I don't think that anything else is using node, apart from a LESS compiler which Gentlecat mentioned, which is presumably for one of the other projects. However, the Lobes/BookBrainz LESS compiling was unaffected by the upgrade to Node 5.x, so there are unlikely to be any issues.

This should be possible by running:

curl -sL https://deb.nodesource.com/setup_5.x | sudo -E bash -
sudo apt-get install -y nodejs