title: RenderAspectRatio has unbounded constraints
body: <p>I'm trying to build a scroll slider in a flutter app using this code to give to build the constraints:</p>
  
  <pre><code>var cardAspectRatio = 12.0 / 16.0;
  var widgetAspectRatio = cardAspectRatio * 1.2;
  
  AspectRatio (
  aspectRatio: widgetAspectRatio,
    child: LayoutBuilder(builder: (context, constraints) {
      var width = constraints.maxWidth;
      var height = constraints.maxHeight;
  })
  </code></pre>
  
  <p>In order to give it its width and height constraints but the resulting error says that the box constraints are unbounded:</p>
  
  <pre><code>I/flutter ( 6957): RenderAspectRatio has unbounded constraints.
  I/flutter ( 6957): This RenderAspectRatio was given an aspect ratio of 
                     0.8999999999999999 but was given both unbounded
  I/flutter ( 6957): constraints: BoxConstraints(unconstrained)
  I/flutter ( 6957): size: MISSING
  I/flutter ( 6957): aspectRatio: 0.9
  </code></pre>
  
  <p>How can i solve this?</p>