fix(firebase_auth): use correct UTC time from server for currentUser?.metadata.creationTime & currentUser?.metadata.lastSignInTime (#9248)

Author: russellwheatley

packages/firebase_auth/firebase_auth/test/user_test.dart

@@ -333,8 +333,8 @@ void main() {
           'uid: 12345)';
 
       final userMetadata = 'UserMetadata('
-          'creationTime: ${DateTime.fromMillisecondsSinceEpoch(kMockCreationTimestamp)}, '
-          'lastSignInTime: ${DateTime.fromMillisecondsSinceEpoch(kMockLastSignInTimestamp)})';
+          'creationTime: ${DateTime.fromMillisecondsSinceEpoch(kMockCreationTimestamp, isUtc: true)}, '
+          'lastSignInTime: ${DateTime.fromMillisecondsSinceEpoch(kMockLastSignInTimestamp, isUtc: true)})';
 
       expect(
         auth!.currentUser.toString(),

packages/firebase_auth/firebase_auth_platform_interface/lib/src/user_metadata.dart

@@ -17,15 +17,15 @@ class UserMetadata {
   /// When this account was created as dictated by the server clock.
   DateTime? get creationTime => _creationTimestamp == null
       ? null
-      : DateTime.fromMillisecondsSinceEpoch(_creationTimestamp!);
+      : DateTime.fromMillisecondsSinceEpoch(_creationTimestamp!, isUtc: true);
 
   /// When the user last signed in as dictated by the server clock.
   ///
   /// This is only accurate up to a granularity of 2 minutes for consecutive
   /// sign-in attempts.
   DateTime? get lastSignInTime => _lastSignInTime == null
       ? null
-      : DateTime.fromMillisecondsSinceEpoch(_lastSignInTime!);
+      : DateTime.fromMillisecondsSinceEpoch(_lastSignInTime!, isUtc: true);
 
   @override
   String toString() {