fix CI to use stable dart 2.12

Author: robrbecker

.github/workflows/dart.yml

@@ -8,7 +8,7 @@ jobs:
     runs-on: ubuntu-latest
 
     container:
-      image: google/dart:2.12-beta
+      image: google/dart:2.12
 
     steps:
     - uses: actions/checkout@v1