title: Determine if function was defined with "async"
body: <p>I'm wondering if there's a way to determine if a function was defined using <code>async function</code> or <code>async (...) =&gt; ...</code> syntax?</p>
  
  <p>I'm looking to implement the function <code>isDefinedWithAsync</code> where:</p>
  
  <pre><code>isDefinedWithAsync(async function() { return null; }) === true;
  isDefinedWithAsync(function() { return null; }) === false;
  isDefinedWithAsync(async () =&gt; null) === true;
  isDefinedWithAsync(() =&gt; null) === false;
  </code></pre>
  
  <p>Is it possible to implement <code>isDefinedWithAsync</code>? And if so, how? Thanks!</p>