title: Strange second argument passed to event handler (always undefined)
body: <p>I've attached event listener with <code>onClick</code> property like this:</p>
  
  <pre><code>&lt;a href="#" onClick={function (event) {
      console.log(arguments.length);   // prints 2
      // arguments[0] = SyntheticEvent - it's ok
      // arguments[1] = undefined - what is this???
  }}&gt;Click me&lt;/a&gt;
  </code></pre>
  
  <p>The event listener called with 2 arguments: synthetic event and strange undefined value.</p>
  
  <p>I think, normal behavior is when handler called with a <em>single</em> param - event object.</p>
  
  <blockquote>
    <p>Windows 10. Chrome 53. React version - 15.2.0.</p>
  </blockquote>