feat: add dart setup action

Author: exaby73

.github/workflows/analyzer.yml

@@ -8,6 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - uses: dart-lang/setup-dart@v1
       - uses: subosito/flutter-action@v2
         with:
           channel: stable