title: get value of selected <select> element in dart
body: <p>how i can obtain selected option of  element?
  document.query("#id") - returns Element which hasn't value propery
  so i can't do:</p>
  
  <pre><code>var val = document.query("#id").value;
  </code></pre>
  
  <p> </p>
  
  <pre><code>var val = (SelectElement)(document.query("#id")).value; // - "SelectElement is class and cannot be used as an expression"
  </code></pre>
  
  <p>how to do type conversion in dart correctly?</p>