remove dev branch of Dart

Author: robrbecker

.github/workflows/dart_ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [ stable, 2.13.4, dev ]
+        sdk: [ stable, 2.13.4 ]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]