title: Index signature where key is string vs [string]
body: <p>What's the difference between these two?</p>
  <pre><code>let foo:{ [index:string] : string } = {
      ['hello']: 'world'
  };
  
  let bar:{ [index:string] : string } = {
      'hello': 'world'
  };
  </code></pre>
  <p>I get the same result (<code>world</code>) when for the two values</p>
  <pre><code>console.log(foo['hello'])
  console.log(bar['hello'])
  </code></pre>