title: flowtype error with react's synthetic event
body: <p>I am actually trying to learn flowtype.
  In my react app I have a reusable TextArea component, this is the props declaration:</p>
  
  <pre><code>type props = {
    fill?: boolean,
    large?: boolean,
    blue?: boolean,
    red?: boolean,
    onChange: (
      e: SyntheticEvent&lt;HTMLInputElement&gt; &amp; { currentTarget: HTMLInputElement },
    ) =&gt; void,
    value: string
  };
  </code></pre>
  
  <p>when I reuse it in an other component:</p>
  
  <pre><code>&lt;TextArea value={this.state.note} onChange={this.changeNote} /&gt;
  </code></pre>
  
  <p>the changeNote method:</p>
  
  <pre><code>  changeNote = (
      e: SyntheticEvent&lt;HTMLInputElement&gt; &amp; { currentTarget: HTMLInputElement },
    ) =&gt; {
      this.setState({ note: e.currentTarget.value });
    };
  </code></pre>
  
  <p>I get this error when running flow:</p>
  
  <pre><code>app/components/pages/bills/BillAddPage.js:256
  256:           &lt;TextArea value={this.state.note} onChange={this.changeNote} /&gt;
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ React element `TextArea`
   21:     e: SyntheticEvent&lt;HTMLInputElement&gt; &amp; { currentTarget: HTMLInputElement },
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ intersection type. Expected polymorphic type instead of. See: app/components/atoms/TextArea.js:21
  141:     e: SyntheticEvent&lt;HTMLInputElement&gt; &amp; { currentTarget: HTMLInputElement },
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ class type: SyntheticEvent
  </code></pre>
  
  <p>What's wrong about that?</p>