validate code coverage on CI (dart-lang/markdown#474)

diff --git a/pkgs/markdown/.github/workflows/test-package.yml b/pkgs/markdown/.github/workflows/test-package.yml
index 4e90396..82b378a 100644
--- a/pkgs/markdown/.github/workflows/test-package.yml
+++ b/pkgs/markdown/.github/workflows/test-package.yml

@@ -59,3 +59,23 @@
       - name: Run VM tests
         run: dart test --platform vm
         if: always() && steps.install.outcome == 'success'
+
+  coverage:
+    needs: test
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@93ea575cb5d8a053eaa0ac8fa3b40d7e05a33cc8
+    - uses: dart-lang/setup-dart@6a218f2413a3e78e9087f638a238f6b40893203d
+      with:
+        sdk: dev
+    - name: Install dependencies
+      run: dart pub get
+    - name: Install coverage
+      run: dart pub global activate coverage
+    - name: Collect and report coverage
+      run: dart pub global run coverage:test_with_coverage
+    - name: Upload coverage
+      uses: coverallsapp/github-action@master
+      with:
+        github-token: ${{ secrets.GITHUB_TOKEN }}
+        path-to-lcov: coverage/lcov.info

diff --git a/pkgs/markdown/README.md b/pkgs/markdown/README.md
index cf6a000..a952014 100644
--- a/pkgs/markdown/README.md
+++ b/pkgs/markdown/README.md

@@ -1,5 +1,7 @@
 [![Build Status](https://github.com/dart-lang/markdown/workflows/Dart%20CI/badge.svg)](https://github.com/dart-lang/markdown/actions?query=workflow%3A"Dart+CI"+branch%3Amaster)
 [![Pub](https://img.shields.io/pub/v/markdown.svg)](https://pub.dev/packages/markdown)
+[![package publisher](https://img.shields.io/pub/publisher/markdown.svg)](https://pub.dev/packages/markdown/publisher)
+[![Coverage Status](https://coveralls.io/repos/github/dart-lang/markdown/badge.svg?branch=master)](https://coveralls.io/github/dart-lang/markdown?branch=master)
 
 A portable Markdown library written in Dart. It can parse Markdown into
 HTML on both the client and server.