title: How to set default value in function
body: <p>I try to set default value in function:</p>
  
  <pre><code>bool isOnGoing([DateTime date = DateTime.now()]) {
      ...
  }
  </code></pre>
  
  <p>But studio returns "Default values of an optional parameter must be constant".</p>
  
  <p>How can I set default parameter in this case?</p>