don't run on dart 3 yet

Author: robbecker-wf

.github/workflows/dart_ci.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        sdk: [ 2.18.7, stable ]
+        sdk: [ 2.18.7, 2.19.6 ]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1