title: SlideTransition in flutter takes space before it slides in
body: <p>I implemented SlideTransition for a widget in flutter, and it slides in as expected. The issue is that before the animation is called, the space where the slider is going to be displayed later is empty.</p>
  
  <p>How can I make the parent to give this space to other widgets in the layout until the moment that a slider comes into view?</p>
  
  <p>I was thinking about giving a slider the initial height of zero, but then the widgets inside the slider would act funny as the height changes in sync with sliding. I wonder if there is a simpler way.</p>
  
  <p>The parent is:</p>
  
  <pre><code>new Scaffold(
      appBar: _appBar,
      body: new Column(
        children: &lt;Widget&gt;[
          new Expanded(
            child: _body;
            }),
          ),
          new SlideTransition(
            position: _sliderPosition, // 
            child: _slider,
          ),
        ],
      );
  </code></pre>
  
  <p>And the position is defined as:</p>
  
  <pre><code>_sliderPosition = new MaterialPointArcTween(
    begin: const Offset(0.0, 1.0),
    end: Offset.zero,
  ).animate(_animationController);
  </code></pre>