try building on macos

.github/workflows/test.yaml

@@ -37,7 +37,7 @@ jobs:
       - run: flutter test --coverage
 
   android:
-    runs-on: ubuntu-latest
+    runs-on: macos-latest
 
     steps:
       - name: Checkout code
@@ -47,7 +47,7 @@ jobs:
       - name: Setup Java
         uses: actions/setup-java@v1
         with:
-          java-version: '8.x'
+          java-version: '12.x'
       - name: Setup Flutter
         uses: subosito/flutter-action@v1
         with: