title: How to declare final class in Dart to prevent extending from it?
body: <p>In Java\Kotlin we have a <code>String</code> class that is final and immutable.</p>
  <p>I tried to mark the class with <code>final</code> keyword but looks like it's not allowable.</p>
  <p>So, I'm a little bit confusing, how to declare final class in Dart ?</p>
  <blockquote>
  <p>Note: the case is - I want to instantiate this class outside, but forbid to extending it. So using the private constructor - it's not my case.</p>
  </blockquote>