@viejo I’m guessing your app was created a while back. For new Meteor apps, the assert built-in package is provided by the meteor-node-stubs npm package. So if you run
meteor npm install --save meteor-node-stubs
then you’ll have access to assert and all the other Node built-ins, though they will be bundled only if you actually use them (so you don’t need to worry about the size of unused packages).