btoa() takes a byte string, so it must decode the UTF-8 argument into
a Vector<u8> before calling encode_base64.
Likewise, in atob() decode_base64 returns a byte string, so that needs
to be converted to UTF-8.
With this, `btoa(String.fromCharCode(255))` is '/w==' as it should
be, and `atob(btoa(String.fromCharCode(255))) == String.fromCharCode(255)`
remains true.