Dashed downward arrow failed in XyJax

Using LaTeX package xypic, the code

$$xymatrix{
&ar@{.}[d]\
1&2
}$$

produces a dashed downward arrow:     texlive

so it is valid xypic code.

But it causes an error in XyJax (an implementation of xypic in Javascript. Examples)

<script>
  MathJax = {
    loader: {
      load: ['[custom]/xypic.js'],
      paths: {custom: 'https://cdn.jsdelivr.net/gh/sonoisa/[email protected]/build/'}
    },
    tex: {
      packages: {'[+]': ['xypic']}
    }
  };
</script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/es5/tex-chtml.js"></script>

$$xymatrix{
&ar@{.}[d]\
1&2
}$$

Error: SyntaxError ‘}’ expected but ‘[‘ found. Parse error at or near “&ar@{.}[d]
“.

I tried to read the source of XyJax parser, but can’t locate code that causes the error.

A temporary workaround: 

<script>
  MathJax = {
    loader: {
      load: ['[custom]/xypic.js'],
      paths: {custom: 'https://cdn.jsdelivr.net/gh/sonoisa/[email protected]/build/'}
    },
    tex: {
      packages: {'[+]': ['xypic']}
    }
  };
</script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/es5/tex-chtml.js"></script>

$$xymatrix{
&{}ar@{.}[d]\
1&2
}$$