Bump @babel/preset-env from 7.7.7 to 7.8.4 in /assets (#639)

Bumps [@babel/preset-env](https://github.com/babel/babel) from 7.7.7 to 7.8.4.
- [Release notes](https://github.com/babel/babel/releases)
- [Changelog](https://github.com/babel/babel/blob/master/CHANGELOG.md)
- [Commits](babel/babel@v7.7.7...v7.8.4)

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