title: Check if a file Exists async?
body: <p>I wish there was a <code>File.ExistsAsync()</code></p>
  
  <p>I have:</p>
  
  <pre><code>bool exists = await Task.Run(() =&gt; File.Exists(fileName));
  </code></pre>
  
  <p>Using a thread for this feels like an antipattern.
  Is there a cleaner way?</p>