CI: analyze with Dart SDK from the master channel

Dart SDK master supports nested structs.

Author: jpnurmi

.github/workflows/test-package.yml

@@ -44,7 +44,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: dart-lang/[email protected]
         with:
-          channel: stable
+          channel: master
       - name: Install dependencies
         run: dart pub get  
       - name: Install libclang-10-dev