Thanks for pointing me in the right direction, got it now 
On the error
ReferenceError: Unknown plugin “transform-object-rest-spread”
I had to install the dependency
sudo npm install babel-plugin-transform-object-rest-spread
After that, the following error occurred on bench build:
Error: Couldn't find preset "env" relative to directory "/home/frappe/frappe-bench/sites"
This was resolved with
sudo npm install babel-preset-env
And now it works Not sure what put the system into this state (vanilla Debian 8 with easy install script production). Again, thanks.