title: What is the full list of provider id's for firebase.UserInfo.providerId?
body: <p>The docs say</p>
  
  <blockquote>
    <p>For example, 'facebook.com', or 'google.com'.</p>
  </blockquote>
  
  <p>However, is the exhaustive list available somewhere and can I depend on it? Can these id's ever be a variation, like 'facebook' or 'google' (without the .com)?</p>
  
  <p>Context: I am trying to figure out if the signed in user has a social login. So I want to loop through <code>currentUser.providerData</code> and match it against a known list of providerIds.</p>