title: Firebase for Flutter sign-in build failure
body: <p>I have been following the Firebase for Flutter tutorial with success until I reach "Set up Firebase Integration". When I relaunch the app on my phone the build fails with the following exception:</p>
  
  <blockquote>
    <p>Note:
    /Users/williamdevore/.pub-cache/hosted/pub.dartlang.org/firebase_storage-0.0.2/android/src/main/java/io/flutter/plugins/firebase/storage/FirebaseStoragePlugin.java
    uses unchecked or unsafe operations. Note: Recompile with
    -Xlint:unchecked for details. /Users/williamdevore/.pub-cache/hosted/pub.dartlang.org/google_sign_in-0.0.6/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java:30:
    error: cannot find symbol import
    io.flutter.app.FlutterFragmentActivity;
                         ^   symbol:   class FlutterFragmentActivity   location: package io.flutter.app
    /Users/williamdevore/.pub-cache/hosted/pub.dartlang.org/google_sign_in-0.0.6/android/src/main/java/io/flutter/plugins/googlesignin/GoogleSignInPlugin.java:98:
    error: cannot find symbol
                  + FlutterFragmentActivity.class.getName());
                    ^   symbol:   class FlutterFragmentActivity   location: class GoogleSignInPlugin 2 errors</p>
    
    <p>FAILURE: Build failed with an exception.</p>
  </blockquote>
  
  <p>Flutter doctor says everything is fine for Android. I have the latest "everything". I was able to setup the google-services json file and use the Firebase console with no problems.</p>
  
  <p>I followed the instructions for modifying the build.gradle files exactly--twice!.</p>
  
  <p>Is there some of other step I should be doing outside of the Flutter tutorial that is specific for Android, perhaps Brew or cocoapods maybe?</p>