title: Fail to compile flutter app with firebase on the last version of both
body: <p>I'm starting with flutter.
  I compiled the Hello World project and it worked.
  But I'm having trouble compiling the project when I enter dependency on firebase_auth.</p>
  
  <p>I followed the steps:</p>
  
  <ol>
  <li>I inserted the latest version of firebase_auth in pubspec.yaml</li>
  </ol>
  
  <pre><code>dependencies:
    firebase_auth: ^0.8.4+2
  </code></pre>
  
  <ol start="2">
  <li>Clicked on Packages get. Successful</li>
  <li>Run 'main.dart'. ERROR!</li>
  </ol>
  
  <pre><code>Initializing gradle...
  Resolving dependencies...
  Running Gradle task 'assembleDebug'...
  /usr/local/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_auth-0.8.4+2/android/src/main/java/io/flutter/plugins/firebaseauth/FirebaseAuthPlugin.java:9: error: cannot find symbol
  import androidx.annotation.NonNull;
                            ^
    symbol:   class NonNull
    location: package androidx.annotation
  /usr/local/flutter/.pub-cache/hosted/pub.dartlang.org/firebase_auth-0.8.4+2/android/src/main/java/io/flutter/plugins/firebaseauth/FirebaseAuthPlugin.java:10: error: cannot find symbol
  import androidx.annotation.Nullable;
  
  ...
  
  BUILD FAILED in 16s
  *******************************************************************************************
  The Gradle failure may have been because of AndroidX incompatibilities in this Flutter app.
  See https://flutter.dev/docs/development/packages-and-plugins/androidx-compatibility for more information on the problem and how to fix it.
  *******************************************************************************************
  </code></pre>
  
  <p>All the tutorials I've seen integrating firebase_auth with flutter worked, however in previous versions.</p>
  
  <p>Is it really an Android compatibility issue?</p>
  
  <p>I searched for hours about the error but found no solution.</p>