title: React.cloneElement -> Add className element
body: <p>Suppose I have a <code>ReactElement</code> that has a className, and I want to add a new class to its className, without overriding the existing className.
  How am I supposed to do this?</p>
  
  <p>I've tried the following but it does override the existing className:</p>
  
  <pre><code>var hello = &lt;Hello name="World" className="base"/&gt;;
  hello = React.cloneElement(hello,{className: "text1"});
  hello = React.cloneElement(hello,{className: "text2"});
  </code></pre>
  
  <p>However this solution works:</p>
  
  <pre><code>var hello2 = &lt;Hello name="World" className="base"/&gt;;
  hello2 = React.cloneElement(hello2,{className: hello2.props.className + " test1"});
  hello2 = React.cloneElement(hello2,{className: hello2.props.className + " test2"});
  </code></pre>
  
  <p>But is this safe to use <code>ReactElement.props</code> like that? Is it part of the public API of a ReactElement and is supposed to be kept retrocompatible in the future? I was not able to find this in the documentation.</p>
  
  <p>Is there another way to achieve this?</p>