title: Firebase Admin SDK GO VerifyIDToken returns not an App Engine context error
body: <p>I have a user authenticated with Firebase and what I want to do is to authenticate this user on the backend(Google cloud platform / Go) as well. I followed along with the document on <a href="https://firebase.google.com/docs/auth/admin/verify-id-tokens" rel="nofollow noreferrer">Firebase</a></p>
  
  <p>I get the idToken on the front end and send the token on the header to the server running on my localhost with the following code.</p>
  
  <pre><code>idToken = firebase.auth().currentUser.getIdToken()
  axios({
      method: 'POST',
      url: 'https://localhost:8080/users',
      headers: {
        'Authentication-Token': idToken
      },
      data: {
          name: 'My name',
          user_name: 'my_user_name',
          email: 'example@example.com'
      }
  })
  </code></pre>
  
  <p>On the backend I want to verify the idToken. I first create the auth client with the following code.</p>
  
  <pre><code>opt := option.WithCredentialsFile("firebase_credential.json")
  app, err := firebase.NewApp(context.Background(), nil, opt)
  client, err := app.Auth(context.Background())
  </code></pre>
  
  <p>With the client and the idToken from the frontend I tried the code below.</p>
  
  <pre><code>t, err := client.VerifyIDToken(context.Background(), idToken)
  </code></pre>
  
  <p>How ever this throws an error of </p>
  
  <pre><code>{"message":"Get https://www.googleapis.com/robot/v1/metadata/x509/securetoken@system.gserviceaccount.com: oauth2: cannot fetch token: Post https://accounts.google.com/o/oauth2/token: not an App Engine context"}
  </code></pre>
  
  <p>I also tried to <code>curl</code> the same request to the sever but the same error came out.</p>
  
  <pre><code>curl -X POST -H "Authentication-Token:hogehogemytoken" -d '{"name":"My name","user_name":"my user name","email":"example@example.com"}' http://localhost:8080/users
  </code></pre>
  
  <p>I am quite sure that I'm sending the right token.
  Any ideas?</p>