title: React.PureComponent not implementing shouldComponentUpdate
body: <p>I might be missing something, but I have a component like this</p>
  
  <pre><code>export default MyComponent extends React.PureComponent {
      // ...
  }
  </code></pre>
  
  <p>When MyComponent is part of another components render method, MyComponent re-renders every time the parent renders, even when the props/state are unchanged. So it seems changing from <code>React.Component</code> to <code>React.PureComponent</code> did not make the component "pure".</p>
  
  <p>I tried adding</p>
  
  <pre><code>console.info(this.shouldComponentUpdate)
  </code></pre>
  
  <p>inside the one of the components methods, and it says it is undefined. Isn't <code>React.PureComponent</code> supposed to add a shallow-comparing <code>shouldComponentUpdate</code> method?</p>
  
  <p>This has now happened with React <strong>15.5.4</strong> and <strong>15.6.0</strong></p>