title: Flow type extends generic type and private member with symbol
body: <p>I'm trying to add flow type to my code and I have some issue with
  Generic types, I want to write something like this <code>T extends Object</code>
  to be able to use <code>obj.constructor.name</code> and so on.
  Also, I have issue with Symbol in class
  please see following code</p>
  
  <pre><code>const postProcessors = new Symbol("postProcessors");
  
  class Container&lt;T extends Object&gt; {
      [postProcessors]: PostProcessor[];
  
      constructor() {
          this[postProcessors] = [];
      }
  
      enablePostProcessor(postProcessor: PostProcessor) {
          this[postProcessors].push(postProcessor);
      }
  
      getInstance&lt;T&gt;(constructor: T): T{
          //...
      }
  }
  </code></pre>