title: Typed unsealed object does not allow new props
body: <p>Why flow does not allow new props to be written into typed <a href="https://flow.org/en/docs/types/objects/#toc-unsealed-objects" rel="nofollow noreferrer">unsealed object</a>:</p>
  <pre><code>type T = {
    a: string;
  };
  
  const t: T = {};
  t.a = 'test'; // ok
  t.b = 22; //      ^ Cannot assign `22` to `t.b` because property `b` is missing in `T` [1]. [prop-missing]
  </code></pre>
  <p>Changing <code>T</code> to inexact type does not help:</p>
  <pre><code>type T = {
    a: string,
    ...
  }
  </code></pre>
  <p><a href="https://flow.org/try/#0C4TwDgpgBAKlC8UDeAoKUCGAuKBnYATgJYB2A5gNxpQB0dKAvlSgMYD2J+UwOciSTFMBoYEUAOTAI+cRSgB6eVDYBrITQBGYgEzaKQA" rel="nofollow noreferrer">Try</a></p>