title: an interface may only extend a class or another interface / mapped types
body: <p>I use <code>typescript 2.1</code> along with <code>JSX</code> and I try to use <code>Partial</code> mapped type for extend interface of react component with optional properties from interface with required properties.</p>
  
  <p>this is interface with required properties:</p>
  
  <pre><code>interface ActionsCreators {
    A: string;
    B: string;
  };
  </code></pre>
  
  <p>this is interface which I want to extend:</p>
  
  <pre><code>interface Props extends React.Props&lt;any&gt; {
    C: string;
  };
  </code></pre>
  
  <p>this is mapped type:</p>
  
  <pre><code>type ActionsCreatorsPartial = Partial&lt;ActionsCreators&gt;;
  </code></pre>
  
  <p>which give this result:</p>
  
  <pre><code>type ActionsCreatorsPartial = {
    A?: string;
    B?: string;
  }
  </code></pre>
  
  <p>this is what I try to do:</p>
  
  <pre><code>interface Props extends React.Props&lt;any&gt;, ActionsCreatorsPartial {
     C: string;
  };
  </code></pre>
  
  <p>but I see compilation error: "an interface may only extend a class or another interface" How can I connect interface <code>Props</code> with <code>ActionsCreatorsPartial</code> type?</p>