ci: use codecov's coverage "carryforward" flags (#1159)

.github/actions/dart_package/action.yaml

@@ -70,6 +70,7 @@ runs:
     - name: Upload Coverage
       uses: codecov/codecov-action@v3
       with:
+        flags: ${{ inputs.working_directory }}
         token: ${{ inputs.codecov_token }}
 
     - uses: VeryGoodOpenSource/very_good_coverage@v2

.github/codecov.yml

@@ -0,0 +1,8 @@
+# Not sure if .yaml suffix would work, currently using .yml to match examples:
+# https://docs.codecov.com/docs/codecov-yaml
+# Validate with:
+# curl -X POST --data-binary @codecov.yml https://codecov.io/validate
+# See https://docs.codecov.com/docs/flags
+flag_management:
+  default_rules: # the rules that will be followed for any flag added, generally
+    carryforward: true