Use Flutter SDK instead of standalone Dart to resolve packages

The issue is that the Dart packages reference Flutter SDK, so we need
Flutter installed to properly resolve dependencies and avoid formatting
differences.

.github/workflows/format-check-dart.yml

@@ -10,22 +10,24 @@ jobs:
     steps:
     - uses: actions/checkout@v3
 
-    - name: Install Dart
-      uses: dart-lang/setup-dart@v1
+    - name: Setup Flutter
+      uses: subosito/flutter-action@v2
       with:
-        sdk: '3.8.1'
+        flutter-version: '3.24.0'
+        channel: 'stable'
 
     - name: Check dart format version
       run: |
         dart format --version
+        flutter --version
         
-    - name: Resolve Dart packages
+    - name: Resolve Flutter packages
       run: |
         echo "=== Resolving packages to avoid format warnings ==="
         cd spine-flutter
-        dart pub get || true
-        cd example && dart pub get || true
-        cd ../test && dart pub get || true
+        flutter pub get
+        cd example && flutter pub get
+        cd ../test && flutter pub get
         
     - name: Format Dart
       run: ./formatters/format-dart.sh