title: Typescript, Class has no index signature
body: <p>A simple implementation of the my code is like this:</p>
  
  <pre><code>interface IndexSignature{
    [key:string]:any;
  }
  class Foo implements IndexSignature{
    bar(){};
    baz(){
      this['bar'](); //Error: Element implicitly has an 'any' type because type 'Foo' has no index signature.
    }
  }
  </code></pre>
  
  <p>how can I add index signature to the Foo class?</p>