title: Flow Cannot assign function to because property abort is not writable
body: <p>I'm using Flow latest version (0.75.0) and I do not find the way to overwrite the <code>abort</code> method without Flow complains</p>
  
  <pre><code>interface Inter {
    abort(): void;
    done(): void;
  }
  
  class Hello implements Inter {
     abort() {}
     done(){}
  }
  
  let h = new Hello();
  
  h.abort = function() {}
  </code></pre>
  
  <p>The error is the following.   </p>
  
  <pre><code>../-:15: h.../ abort = function() {}
             ^ Cannot assign function to `h.abort` because property `abort` is not writable.
  </code></pre>
  
  <p><a href="https://flow.org/try/#0JYOwLgpgTgZghgYwgAgJLmsg3gKGcgajgCMB7KMALmQDEBXEBMYUkAbj0IBNWJr7GzVhwC+OcQgA2cAM4zkACQiTJpZMAC2AB0kQNEcPPSQo2cfmQlyYABQBKbGIs8QEe1jFicusMgAWyAC8yK4A7orKqvYcOH4AdFYUQcgwDEwsIPaOQA" rel="noreferrer">https://flow.org/try/#0JYOwLgpgTgZghgYwgAgJLmsg3gKGcgajgCMB7KMALmQDEBXEBMYUkAbj0IBNWJr7GzVhwC+OcQgA2cAM4zkACQiTJpZMAC2AB0kQNEcPPSQo2cfmQlyYABQBKbGIs8QEe1jFicusMgAWyAC8yK4A7orKqvYcOH4AdFYUQcgwDEwsIPaOQA</a></p>