title: Property 'set' does not exist on type Set
body: <p>I got the following error: <code>TS2339: Property 'set' does not exist on type Set&lt;&gt;</code>
  I have the following code: </p>
  
  <pre><code>const id = 123;
  const a: Map&lt;string, Set&lt;File&gt;&gt; = new Map();
  if (a.has(id)) {
      a.set(id, new Set());
      a.get(id).set(value);
  }
  </code></pre>