title: How to allow class functions to be nullable
body: <p>I want to allow the class function to be nullable(In short support something like this):</p>
  <pre><code>class Example {
      a?: string;
      c?: string;
      b() {
          console.log(this.a);
      }
  }
  
  let c = (x: Example) {
      console.log('run');
  }
  c({a: &quot;123&quot;});
  </code></pre>
  <p><strong>Background</strong>:
  I've added some presave hook for typeorm due to which most of the code has broken. I'd like to support it without breaking anything.</p>
  <p><a href="https://www.typescriptlang.org/play?#code/MYGwhgzhAECiAeYC2AHEBTaBvAUNf0YA-AFzQQAuATgJYB2A5gNx4HCnnX3Ov4BGACgCU2XgXzAA9nQiSMAOhCSGAigAsaEeWCEtxAXxyGcGCtGDQAvNAHwyCZGnQjc4qTLnpFygQHIqAK50vrpGOMACWGBkAEQAjABMAMwx%20rpAA" rel="nofollow noreferrer">Play link</a></p>