title: d3.js v4 d3.layout.tree has been removed?
body: <p>After upgrading to d3.js v4.1.1, this line:</p>
  
  <pre><code>d3.layout.tree()
  </code></pre>
  
  <p>produces an error:</p>
  
  <pre><code>Cannot read property 'tree' of undefined
  </code></pre>
  
  <p>It seems like the tree layout has been removed from v4? <a href="https://github.com/d3/d3/blob/master/API.md" rel="noreferrer">https://github.com/d3/d3/blob/master/API.md</a></p>
  
  <p>The examples still use the v3 API:
  <a href="http://bl.ocks.org/mbostock/1093025" rel="noreferrer">http://bl.ocks.org/mbostock/1093025</a></p>
  
  <p>Is the layout really gone or has it been renamed?</p>