title: React animation not working
body: <p>I have a <code>Notification</code> class to manage creating/removing Notifications from the UI. I can successfully show/hide notifications, but the notifications do not animate in/out.</p>
  
  <pre><code>import React from 'react'
  import addons from 'react/addons'
  
  const ReactCSSTransitionGroup = React.addons.CSSTransitionGroup
  
  let id = 0
  export default class Notification {
  
    constructor (content, className) {
  
      let div = document.createElement('div')
      document.body.appendChild(div)
  
      let onClose = () =&gt; React.unmountComponentAtNode(div)
  
      React.render(
        &lt;NotificationElement className={ className } id={ id++ } onClose={ onClose }&gt;
          { content }
        &lt;/NotificationElement&gt;,
        div
      )
  
    }
  
  }
  
  class NotificationElement extends React.Component {
  
    constructor(_) {
      super(_)
    }
  
    render() {
  
      const className = `Notification ${ this.props.className }`
  
      return (
        &lt;ReactCSSTransitionGroup transitionName="slideIn"&gt;
          &lt;div className={ className } key={ this.props.id }&gt;
            { this.props.children }
            &lt;a className="close-button" onClick={ this.props.onClose }&gt;&amp;times;&lt;/a&gt;
          &lt;/div&gt;
        &lt;/ReactCSSTransitionGroup&gt;
      )
  
    }
  
  }
  </code></pre>