use dart 2.12-dev

Author: hoc081098

.github/workflows/dart.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
 
     container:
-      image:  google/dart:latest
+      image:  google/dart:2.12-dev
 
     steps:
     - uses: actions/checkout@v2