title: Polymer 3 lit-element: Array Object changes not picked up by requestRender
body: <p>I have an object as below. Now when I change a property and call requestRender it won't trigger. As far as I see it, the property change is not detected. How can I achieve this?
  <div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>static get properties() {
    return {
      items: Object
      }
  }
  
  constructor() {
    super();
    this.items = {
      list: [{ a: 5, b: 6 },{ a: 5, b: 6 }]
    }
  }
  
  onClick() {
    this.items.list[1].a = 10; //change property
    requestRender(); //doesn't pick up the change...
  }</code></pre>
  </div>
  </div>
  </p>