Merge new dartdoc binary to 1.12

[mit-mit]

@devoncarew, we need to merge the new dartdoc binary (dart-lang/dartdoc#620) into the 1.12 dev branch, right? If so, can you please add the MergeToDev label to this bug once ready as per the cherry-pick process?

[devoncarew]

I believe @keertip is working on this.

[mit-mit]

Closing this, 24131 is tracking the merge