For example, the following should trigger a test error:

await tester.pumpWidget(
  MaterialApp(
    home: Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          const Text(
            'You have pushed the button this many times:',
          ),
          Text(
            '5',
            style: Theme.of(context).textTheme.headline4,
          ),
          Image.network('https://via.placeholder.com/150'),
        ],
       ),
    ),
  ),
);

The error should tell the developer that they should either provide a semantic label for the image, or exclude it from semantics. Otherwise screen readers may focus on the image and not have an appropriate thing to read out, which is frustrating for a11y users.

I think we should do this in testWidgets, with a walk of the a11y tree that checks for unlabeled/hinted/valued leaf nodes. It could be done as an a11y matcher, but ideally this is something tests would get opted into to catch errors that developers tend to miss in this regard.

/cc @goderbauer @chunhtai