title: How to use Object.observe on Map?
body: <p>How to use <a href="https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Object/observe" rel="nofollow noreferrer">Object.observe</a> on Map?
  This doesn't work:</p>
  
  <pre><code>var map = new Map();
  Object.observe(map, function() { console.log("ok"); });
  map.set('key', 'value');
  </code></pre>
  
  <p>And this too:</p>
  
  <pre><code>var map = new Map();
  Array.observe(map, function() { console.log("ok"); });
  map.set('key', 'value');
  </code></pre>