title: Angular2 find does not exist on Array
body: <p>I have an angular2 project (2.0.0)  using typescript 2.0.4.  </p>
  
  <p>I have this code:</p>
  
  <pre><code>var assignedIds: Array&lt;string&gt; = this.AssignedUsers.map(u =&gt; u.id);
  var users = this.AllUsers.filter(all =&gt; all.id != assignedIds.find(id =&gt; id == all.id));
  </code></pre>
  
  <p>I'm getting an error on the find method:</p>
  
  <blockquote>
    <p>Property 'find' does not exist on type 'string[]'.</p>
  </blockquote>
  
  <p>I get this error on several arrays - not just this one.  A couple are number[], one is string[], and one it a custom object.</p>
  
  <p>If I compile to es6 it works but I need to target es5.</p>
  
  <p>Do I need some typings?</p>