title: Check if a file exists before loading it
body: <p>What I want to do is load an image in a Material Widget to use it in a ListTile, but this asset might not exist.</p>
  <pre><code>class MyImage extends StatelessWidget {
    final imagePath;
  
    MyIcon(String iconName) {
      try { // check if imagePath exists. Here is the problem
        imagePath = check('assets/$iconName.png/');
      } catch (e, s) { // if not
        imagePath = 'assets/$iconName.png/';
      }
    }
  
   @override
    Widget build(BuildContext context) {
      return Material(...here I will load the imagePath...);
   }
  }
  </code></pre>
  <p>So, since I'm using a Stateless widget, I have to know beforehand if the image exists, otherwise I'll load a null right?</p>