title: Google OAuth2 JS can't get Authorization Code
body: <p>I have a custom Google Sign In button in React (follows branding guidelines) that I need to implement into a web app. The android app already has Google Sign In implemented with the BE and requires the authorization code to be sent to the BE. I seem to have trouble accessing the authorization code (I can get the access token, id, and id_token fairly easily though).</p>
  
  <p>I initialize as such:</p>
  
  <pre><code>static googleInit (googleSignInID) {
      gapi.load('auth2', () =&gt; {
          ThirdPartyLogInApi.auth2 = gapi.auth2.init({
              // Retrieve the singleton for the GoogleAuth library and set up the client.
              client_id: `${googleSignInID}.apps.googleusercontent.com`,
          })
      })
  }
  </code></pre>
  
  <p>In my React component, I attach a click handler to a React ref (as I've seen in other stackoverflow problems).</p>
  
  <pre><code>componentDidMount () {
      // element, options, success, failure
      ThirdPartyLogInApi.auth2.attachClickHandler(
          this.button,
          {},
          this.onGoogleSuccess,
          this.setProcessingToFalse
      );
  }
  </code></pre>
  
  <p>In the success callback (this.onGoogleSuccess), I am able to call googleUser.getBasicProfile() to get user information and googleUser.getAuthResponse(true) to get id_token and access_token, but I need the authorization code too.</p>
  
  <p>I'm at my wits end with Google's documentation on this - can someone point me in the right direction? I don't want to switch the entire auth flow to use gapi.auth2.authorize, as I do need the user information.</p>
  
  <p>Thanks</p>
  
  <p>EDIT: Is it even possible to access the authorization code? It seems like Google abstracts away that step and just returns the id_token right away. I just need something that looks like:
  4/RQFWJLGd3sJQrgxuRaguzJy2yiUV4fAqVGftRSUYuqc</p>