title: Array.prototype.includes on Node js versions <= 4
body: <p>I had written a Gruntfile, which makes heavy use of Array.prototype.includes() and similar functions.  I have discovered I need to downgrade the version of node to version 4.4.5.  Once I do this, I no long am able to use statements such as  <code>if ( myarray.includes(somevalue) )</code>, and it will fail saying: <code>&gt;&gt; TypeError: myarray.includes is not a function.</code>  When I look at the node documentation, it seems to be for the current version of node, so I'm not sure what's the alternative.</p>
  
  <p>What is the equivalent of an array 'includes', in versions of node 4 and below?  Also, are there other huge differences I'd need to be aware of?  (Another I have found is no support for default params in a function declaration).</p>