Created by: drify
In https://github.com/mathjax/MathJax/commit/65685f43415ec96ef43d9038e40ec25abcb5fc22 mhchem extension under the unpacked/ directory has been updated, but the packed version hasn't yet. As a consequence, if legacy: false is specified, we don't get the latest version.
This version was copied from MathJax-mhchem/mhchem.js