One of the packages relies on a package, 'hex-rgb'. This package uses ES6 functionality, however uglify does not support ES6. I'd expect babelify to do something about this but... -shrugs-
Easiest fix that probably doesn't break anything; change the package from 'uglify-js' to 'uglify-es', which is made by the same author but supports ES6 expressions.
One of the packages relies on a package, 'hex-rgb'. This package uses ES6 functionality, however uglify does not support ES6. I'd expect babelify to do something about this but... -shrugs-
Easiest fix that probably doesn't break anything; change the package from 'uglify-js' to 'uglify-es', which is made by the same author but supports ES6 expressions.