ci: add dartfmt lint

.github/workflows/lint.yml

@@ -0,0 +1,20 @@
+name: lint
+on:
+  push:
+    branches: [master]
+  pull_request:
+    branches: [master]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    container:
+      image: google/dart:latest
+    steps:
+      - uses: actions/checkout@v2
+      - run: pub get
+      - run: \
+        output=$(dartfmt --dry-run lib/**/*.dart)
+        if [ -z $output]
+          then exit 0
+        else
+          exit 1