title: Using Promise as return type in Flow
body: <p>I'm using Flow 0.30.0.  I have a function that returns a promise:</p>
  
  <pre><code>function process(callback:Function):Promise {
    return new Promise((r,re) =&gt; callback)
  }
  </code></pre>
  
  <p>Flow raises an error, complaining:</p>
  
  <pre class="lang-none prettyprint-override"><code>function process(callback:Function):Promise {
                                      ^^^^^^^ Promise. Application of polymorphic type needs &lt;list of 1 argument&gt;. (Can use `*` for inferrable ones)
  </code></pre>
  
  <p>This works in <a href="https://flow.org/try/#0GYVwdgxgLglg9mABABwE5wgUwM7YBQQCGANsQEaEQDWAXAGLjTxgCUNACugLYzaaIBvAFCJEqTFBCokYTAHdEnODz548qADTiWiALwA+REVIVqLIQF8gA" rel="noreferrer">Try Flow</a>, but not locally.  Any ideas of the issue?</p>