title: Does a polyfill exist for getDestinationInsertionPoints?
body: <p>I've come across a situation where it would be helpful to know whether a given node has been distributed into a <code>content</code> node. There's a native Shadow DOM function node.getDestinationInsertionPoints() that's very helpful for this purpose, but I don't see a version of this in the polyfill world.</p>
  
  <p>I can work around the need for this by doing my own inspection of a node's parent chain, but it'd be nicer to rely on the standard (?) getDestinationInsertionPoints() function. Does -- or will -- the Shadow DOM polyfill support this in older browsers?</p>