Reference Link Labels Cannot Contain Brackets

[mjordan56]

The markdown package incorrectly parses reference link labels containing brackets.

Both the CommonMark and GitHub Flavored Markdown specifications note that "Link labels cannot contain brackets, unless they are backslash-escaped" (click on the document links to go to the examples in the specifications). The example from both specs show the example Markdown input data:

[foo][ref[]

[ref[]: /uri

should produce the following HTML output:

<p>[foo][ref[]</p>
<p>[ref[]: /uri</p>

However, the markdown package does not properly parse this reference link label. The following sample code:

import 'package:markdown/markdown.dart';

void main() {
  print(markdownToHtml('[foo][ref[]\n\n[ref[]: /uri'));
}

Produces the following incorrect HTML output:

<p><a href="/uri">foo</a></p>