Bump markdown to 49eefd211e7840bac7e11257cd966435ae3cb07f

Changes:
```
> git log --format="%C(auto) %h %s" 93d0eee..49eefd2
 https://dart.googlesource.com/markdown.git/+/49eefd2 Refactor AutolinkExtensionSyntax (#471)
 https://dart.googlesource.com/markdown.git/+/07e2683 Optimise TableSyntax (#472)
 https://dart.googlesource.com/markdown.git/+/9b61871 Make helper class private that should not have been exposed (#476)
 https://dart.googlesource.com/markdown.git/+/299964e Return list for link nodes creation (#452)
 https://dart.googlesource.com/markdown.git/+/aee6a40 validate code coverage on CI (#474)
 https://dart.googlesource.com/markdown.git/+/88f3f8a Fix html entity and numeric character references (#467)

```

Diff: https://dart.googlesource.com/markdown.git/+/93d0eee771f6355be6737c2a865f613f6b105bf1~..49eefd211e7840bac7e11257cd966435ae3cb07f/
Change-Id: I2a88d7c386f567738226701be4edcd7c4818744f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266760
Auto-Submit: Devon Carew <[email protected]>
Commit-Queue: Oleh Prypin <[email protected]>
Reviewed-by: Oleh Prypin <[email protected]>

DEPS

@@ -139,7 +139,7 @@ vars = {
   "linter_rev": "f2c55484e8ebda0aec8c2fea637b3bd5b17258ca", # 1.28.0
   "lints_rev": "16bdefe1df529262a6596f79e91003ddbdbd3890",
   "logging_rev": "f322480fb9d9e83e677c08db6d09067059f7ff74",
-  "markdown_rev": "93d0eee771f6355be6737c2a865f613f6b105bf1", # b/255935594
+  "markdown_rev": "49eefd211e7840bac7e11257cd966435ae3cb07f",
   "markupsafe_rev": "8f45f5cfa0009d2a70589bcda0349b8cb2b72783",
   "matcher_rev": "6a9b83bbd73e50df2058b3e8e4aa301df49569c6",
   "mime_rev": "bf041aa372a27aae6f94e185aa0af3932b9de98b",