Android X Migration changes wasn't reverted for firebase_auth ^0.8.0

[kroikie]

Main

Migrate to AndroidX broke firebase_auth for older versions of Android (ex. 6.0);
"Revert AndroidX changes for 1.0.0 plugins (flutter/plugins#1127)" didn't include firebase_auth.
I can't migrate to Android X, because it doesn't support older android versions.

1. Update firebase_auth to ^0.8.0.
2. Gradle fails to compile.

Logs

Launching lib\main.dart on Android SDK built for x86 64 in debug mode...

registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
registerResGeneratingTask is deprecated, use registerGeneratedResFolders(FileCollection)
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\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
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:10: error: cannot find symbol
import androidx.annotation.Nullable;
                          ^
  symbol:   class Nullable
  location: package androidx.annotation
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:638: error: cannot find symbol
  private void reportException(Result result, @Nullable Exception exception) {
                                               ^
  symbol:   class Nullable
  location: class FirebaseAuthPlugin
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:550: error: cannot find symbol
    public void onComplete(@NonNull Task<AuthResult> task) {
                            ^
  symbol:   class NonNull
  location: class FirebaseAuthPlugin.SignInCompleteListener
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:569: error: cannot find symbol
    public void onComplete(@NonNull Task<Void> task) {
                            ^
  symbol:   class NonNull
  location: class FirebaseAuthPlugin.TaskVoidCompleteListener
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:587: error: cannot find symbol
    public void onComplete(@NonNull Task<SignInMethodQueryResult> task) {
                            ^
  symbol:   class NonNull
  location: class FirebaseAuthPlugin.GetSignInMethodsCompleteListener
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:186: error: cannot find symbol
                      public void onComplete(@NonNull Task<AuthResult> task) {
                                              ^
  symbol: class NonNull
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:445: error: cannot find symbol
              public void onComplete(@NonNull Task<GetTokenResult> task) {
                                      ^
  symbol: class NonNull
C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java:499: error: cannot find symbol
          public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                                          ^
  symbol: class NonNull
Note: C:\Google\flutter\.pub-cache\hosted\pub.dartlang.org\firebase_auth-0.8.0\android\src\main\java\io\flutter\plugins\firebaseauth\FirebaseAuthPlugin.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
9 errors

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':firebase_auth:compileDebugJavaWithJavac'.
> Compilation failed; see the compiler error output for details.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights.

* Get more help at https://help.gradle.org

BUILD FAILED in 35s
Gradle task assembleDebug failed with exit code 1
Exited (sigterm)