Linter: run coverage in github actions

[pq]

See: #2315.

[pq]

Setup a little differently than ours: https://medium.com/flutter-community/testing-dart-packages-with-github-actions-4c2c671b1e34 but maybe worth cribbing.

(NOTE: COVERALLS_TOKEN is set in secrets.)

[pq]

@athomas, looking over at https://github.com/dart-lang/dartdoc/pull/2464, I see you helped migrate dartdoc but there were maybe some caveats... Could you advise here? Or would you be willing to help?

[athomas]

Sure, shouldn't be too hard. Take a look at the pkg/coverage workflow.

The Actions part is fairly simple. I've tightened the branch protection settings so that you may use the GITHUB_TOKEN-style coveralls integration used elsewhere. You will not need the COVERALLS_TOKEN anymore.

You will also have to remove this from travis.sh because it will stop working when we're off Travis completely:

    echo "Running coveralls..."
    gem install coveralls-lcov # <-- this gem is Travis specific (will work now but break later)
    coveralls-lcov var/lcov.info

[pq]

Thanks @athomas! I started playing around and hit a quick snag. I bet it's an easy fix? #2432...