EDIT: This answer is outdated. With things like Yarn and NPM 5’s lockfiles it is now easier to ensure you’re dependencies are correct on platforms like Heroku
I had a similar issue related to node_modules being modified somehow locally but the change was not reflect on Heroku, causing my app to crash. It’s relatively easy fix if this is your issue:
# Remove node_modules
rm -fr node_modules
# Reinstall packages
npm i
# Commit changes
git add node_modules
git commit -m 'Fix node_modules dependencies.'
git push heroku master
Hope that helps for others with a similar issue.