title: ValueNotifier does not notify listeners when modifying a list
body: <p>I'm trying to rebuild list when an array changes:</p>
  <pre class="lang-dart prettyprint-override"><code>ValueNotifier&lt;List&lt;String&gt;&gt; numbers = ValueNotifier(['123']);
  numbers.addListener(() =&gt; print(numbers));
  numbers.value.add('456');
  </code></pre>
  <p>However listener isn't called.</p>