title: How to know how many views are in the Navigator?
body: <p>I want to know how many views are in the 'Navigator', but <code>Navigator().pages.length</code> has no effect,</p>
  <p>I found that: 'To use this API, an [onPopPage] callback must also be provided to properly', and I did that <code>Navigator(onPopPage: callback).pages.length</code>, it has no effect yet.</p>