title: Flutter banner does not fit
body: <p>I have a problem with banner widget. To demonstrate it I have made some demonstration code. What I want is to have a banner in the top right corner of a container, but it is ugly, as it overflows its child (pls. see the attached image).</p>
  
  <p><a href="https://i.stack.imgur.com/7Z62w.png" rel="noreferrer"><img src="https://i.stack.imgur.com/7Z62w.png" alt="enter image description here"></a></p>
  
  <p>Here is my code:</p>
  
  <pre><code>class TestPage extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      return Scaffold(
        body: Center(
          child: Banner(
            message: "hello",
            location: BannerLocation.topEnd,
            color: Colors.red,
            child: Container(
              margin: const EdgeInsets.all(10.0),
              color: Colors.yellow,
              height: 100,
              child: Center(child: Text("Hello, banner!"),),
            ),
          ),
        ),
      );
    }
  }
  </code></pre>
  
  <p>I tried to play with the margin, but I still have this behavior even if margin set to 0.</p>
  
  <p>How can avoid this 'banner overflow'?</p>
  
  <p>Thanks a lot!</p>