jsnext:main and module values in package.json are incorrect and can cause build breakage. Previously they did not cause a problem because they were pointing to invalid values and essentially not used.
Since both point to modules that require more than ES2015 syntax, I believe the simplest solution is both should be removed then tools like babel/webpack will use dist/ instead which is already transpiled.
Alternately transpile the module file.
jsforum/jsforum#5
https://shuheikagawa.com/blog/2017/01/05/main-jsnext-main-and-module/