title: PIXIJS eventSystem is not found in V6.0.2
body: <p>I am building a demo in PIXIJS v6.0.2. I am trying to add on click event listeners to some buttons. But that doesn't work, so I went into the documentation and found this snipped of code in their example of a click event.</p>
  <pre><code>// In this a example, you can click on a bunny and the more you click at
  // a time the bigger it becomes. If you click outside of the bunny or
  // lose your clicking speed, the bunny resets.
  
  // Disable interaction plugin (for PixiJS 6)
  // eslint-disable-next-line no-underscore-dangle
  delete PIXI.Renderer.__plugins.interaction;
  
  // Create app
  const app = new PIXI.Application({
      antialias: true,
      autoDensity: true,
      backgroundColor: 0x1099bb,
      resolution: devicePixelRatio,
  });
  document.body.appendChild(app.view);
  
  // Install EventSystem, if not already
  // (PixiJS 6 doesn't add it by default)
  if (!('events' in app.renderer)) {
      app.renderer.addSystem(PIXI.EventSystem, 'events');
  </code></pre>
  <p>But I get this error <code>Uncaught TypeError: ClassRef is not a constructor at Renderer.addSystem (core.js:10214) at Module../src/app.js (app.js:22) at __webpack_require__ (bootstrap:19) at main.js:54738 at main.js:54754 at main.js:54941</code></p>
  <p>I am importing PIXIJS into my file I am doing a webpack/babel build. I'm not sure what I'm missing or doing wrong but I cannot seem to find any answers to this for PIXI 6. I don't even get a response on the PIXI Gitter. Any help is appreciated thank you!</p>