title: Image.network loadingBuilder's loading progress event is null
body: <p>I'm trying to use the <a href="https://api.flutter.dev/flutter/widgets/Image/Image.network.html" rel="noreferrer"><code>Image.network</code></a> constructor's <code>loadingBuilder</code> argument.</p>
  
  <p>While the image loads, I want to show a <code>CircularProgressIndicator</code> with its value set to the ratio of downloaded divided by the expected file size. I expected I could do this using the <a href="https://api.flutter.dev/flutter/painting/ImageChunkEvent-class.html" rel="noreferrer"><code>ImageChunkEvent</code></a> parameter's <code>cumulativeBytesLoaded</code>, <code>expectedTotalBytes</code>.</p>
  
  <p>I found a <a href="https://api.flutter.dev/flutter/widgets/Image/loadingBuilder.html" rel="noreferrer">sample</a> in the documentation:</p>
  
  <blockquote>
    <p>The following sample uses <code>loadingBuilder</code> to show a <code>CircularProgressIndicator</code> while an image loads over the network.</p>
  </blockquote>
  
  <pre><code>Image.network(
    'https://example.com/image.jpg',
    loadingBuilder: (BuildContext context, Widget child, ImageChunkEvent loadingProgress) {
      if (loadingProgress == null)
        return child;
      return Center(
        child: CircularProgressIndicator(
          value: loadingProgress.expectedTotalBytes != null
              ? loadingProgress.cumulativeBytesLoaded / loadingProgress.expectedTotalBytes
              : null,
        ),
      );
    },
  ),
  </code></pre>
  
  <p>But the issue is that the <code>loadingProgress</code> is always <code>null</code>, so the whole <code>loadingBuilder</code> charade I'm doing is for nothing.</p>
  
  <p>Why is <code>ImageChunkEvent loadingProgress</code> not passed to the <code>loadingBuilder</code> function with real values?</p>