title: How can Flow be forced to cast a value to another type?
body: <p>Is it possible to forcibly cast a variable in <a href="https://flowtype.org" rel="noreferrer">Flow</a>?</p>
  
  <pre class="lang-default prettyprint-override"><code>type StringOrNumber = string | number
  const foo: StringOrNumber = 'hello'
  
  // I look for something like `const bar:string = (string) foo`
  const bar: string = foo // fails
  const bar: string = (foo: string) // also fails
  </code></pre>