Doc references with multiple underscores are not recognized as links

[sethladd]

For example:

/// Testing [SOME_BIG_NAME]
void foo() {}

The SOME_BIG_NAME won't be recognized as a link.

Blocked by dart-lang/tools#1481

[sethladd]

Update: dart-lang/tools#1481 was fixed (yay!) so once it lands in pub, we can fix this fairly easily.

And, the new markdown package was published. We can proceed with a fix.

[sethladd]

Addressed by #784