title: How can I layout widgets based on the size of the parent?
body: <p>Let's say you have a parent widget that might have a variable size.</p>
  <p>For example:</p>
  <pre><code>var container = new Container(
     height: 200.0, // Imagine this might change
     width: 200.0, // Imagine this might change
     // Imagine content in this container will 
     // depend on the parent container
     child: new Container(), 
  );
  </code></pre>
  <p>And maybe you want to have the child of the parent container render something different based on the size that it's given.</p>
  <p><em>Think of responsive design breakpoints, if the width is over X use this layout, if the width is under X use that layout.</em></p>
  <p>What's the best way to do this in Flutter?</p>