Add Dart 2.19 to GHA matrix

Author: greglittlefield-wf

.github/workflows/dart_ci.yml

@@ -16,7 +16,7 @@ jobs:
       fail-fast: false
       matrix:
         # Can't run on `stable` (Dart 3) until we're fully null-safe.
-        sdk: [2.18.7]
+        sdk: [2.18.7, 2.19.6]
     steps:
       - uses: actions/checkout@v2
       - uses: dart-lang/setup-dart@v1