title: Make CachedNetworkImage scale to fit height of ListTile
body: <p>I want to place a <code>CachedNetworkImage</code> as the <code>leading</code> property inside a <code>ListTile</code> and have the image scale itself to fit the height of the <code>ListTile</code>. Is this possible without specifying a fixed height for the <code>CachedNetworkImage</code>? </p>
  
  <p>What I have right now is this, which looks good but uses a fixed height of 32px. </p>
  
  <pre><code>return ListTile(
      title: Text(record.title),
      subtitle: Text(record.type),
      leading: CachedNetworkImage(imageUrl: url, fit: BoxFit.scaleDown, height: 32),
      onTap:_onListItemTap,
  );
  </code></pre>
  
  <p>I'm not a fan of fixed dimensions, but perhaps I should be? Can I expect that 32px in Flutter will always be appropriate for a <code>ListTile</code> on any device? </p>
  
  <p>Or is there a better way to make the size of my images adjust dynamically to the other content of the <code>ListTile</code>?</p>