title: Flutter secondaryAnimation not running
body: <p>I am pushing a page called <code>SecondPage</code> onto the stack with the following code. The animation using <code>animation</code> runs as expected. When <code>SecondPage</code> appears, it slides up from the bottom of the screen.</p>
  
  <p>When I'm looking at <code>SecondPage</code>, I push a <code>ThirdPage</code> on top of <code>SecondPage</code>. As I watch <code>ThirdPage</code> appear on the screen, I don't see <code>SecondPage</code> slide up as I expected based on the use of <code>secondaryAnimation</code> below. In fact, there is no animation of <code>SecondPage</code> at all during the transition to <code>ThirdPage</code>. <code>SecondPage</code> is just stationary while being covered by <code>ThirdPage</code>.</p>
  
  <p>How do I get <code>SecondPage</code> to move up while <code>ThirdPage</code> appears?</p>
  
  <pre><code>Navigator.push(
    context,
    PageRouteBuilder(
      pageBuilder: (BuildContext context, Animation&lt;double&gt; animation, Animation&lt;double&gt; secondaryAnimation) {
        return SecondPage();
      },
      transitionsBuilder: (BuildContext context, Animation&lt;double&gt; animation, Animation&lt;double&gt; secondaryAnimation, Widget child) {
        return SlideTransition(
          position: Tween&lt;Offset&gt;(
            begin: const Offset(0.0, 1.0),
            end: Offset.zero,
          ).animate(animation),
          child: SlideTransition(
            position: Tween&lt;Offset&gt;(
              begin: Offset.zero,
              end: const Offset(0.0, -1.0),
            ).animate(secondaryAnimation),
            child: child,
          ),
        );
      },
    ),
  );
  </code></pre>