title: Flow, how to type cast to a more specific type from mixed
body: <p>The flow-typed libdef for express defines <code>locals</code> as an object with <code>mixed</code> values.<br>
  Specifying the actual value types results in the errors below... how should these specific types be annotated?</p>
  
  <pre><code>// From libdef
  type Locals = {
    [name: string]: mixed;
  }
  
  // Populated data
  const locals: Locals = {
    version: 1.2,
    resources: {
      a: "abc",
      b: "def"
    }
  };
  
  // Annotate the actual type
  const version: number = locals.version; // Error: mixed is incompatible with number
  const resources: {[string]: string} = locals.resources; // Error: mixed is incompatible with object type
  </code></pre>