title: how to define ref type by flowtype?
body: <p>I trying to define type of ref by flowtype in react v16.4.0<br>
  But I couldn't resolve it, so please let me know how to define it.  </p>
  
  <p>these are sample code.<br>
  I want to know how to define ref of Props type.  </p>
  
  <h2>Parents</h2>
  
  <pre><code>export default class ParentComponent extends React.Component&lt;Props, State&gt; {
    constructor(props: Props) {
      super(props);
      ....
      this.listRef = createRef();
    }
  
    render() {
      return (
        &lt;ChildComponent
          listRef={this.listRef}
        /&gt;
      );
    }
  }
  </code></pre>
  
  <h2>child</h2>
  
  <pre><code>type Props = {
    listRef: Ref&lt;&gt;, // how to define here?
  };
  
  const ChildComponent = (props: Props) =&gt; {
    &lt;div&gt;
      &lt;ul ref={props.listRef}&gt;
      ...
      &lt;/ul&gt;
      ...
    &lt;/div&gt;
  }
  </code></pre>
  
  <h1>modules version</h1>
  
  <pre><code>"react": "^16.4.0",
  "flow-bin": "^0.73.0",
  </code></pre>