chore: Add reference to dart TT client

[CelticMajora]

Summary

Links out to the newly-published dart TT client

/domain @Betterment/test_track_core @samandmoore @btrautmann /no-platform

[nanda-prbot]

Needs somebody from @Betterment/test_track_core, @samandmoore, and @btrautmann to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}