title: How do I define a private final field that is initialized using an optional constructor argument with a default value?
body: <p>Suppose I have this situation:</p>
  
  <pre class="lang-dart prettyprint-override"><code>class Foo {
    final _bar;
    Foo([bar = 'hello']);
  }
  </code></pre>
  
  <p>In Dart, how do I initialize <code>_bar</code>, given that it's private?</p>