title: Property 'OAuthProvider' does not exist on type 'typeof auth'
body: <p>i am trying for apple login in my ionic 3 project, i got success response on apple login after i tried firebase auth its shows error , please help me to resolve.</p>
  <pre><code> const provider = new firebase.auth.OAuthProvider('apple.com');
  </code></pre>
  <p><strong>Property 'OAuthProvider' does not exist on type 'typeof auth'.</strong></p>
  <p>Thanks</p>