Extend Immutable.js types
<p>I would like to extend functionality of Map class. For example, I want to do following:</p>

<pre><code>const someMap = Immutable.Map({ a: 1, b: 2 });
const {a, b} = someMap.batchGet('a,' 'b');
</code></pre>

<p>How can I correctly add 'batchGet' method to Map class?</p>