Ignore several import_of_legacy_library_into_null_safe (#69904)

* Ignore import_of_legacy_library_into_null_safe instead of un-migrating.

packages/flutter_driver/test/src/real_tests/stubs/stub_command.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter_driver/driver_extension.dart';
-import 'package:flutter_driver/flutter_driver.dart';
+import 'package:flutter_driver/flutter_driver.dart'; // ignore: import_of_legacy_library_into_null_safe
 
 class StubNestedCommand extends CommandWithTarget {
   StubNestedCommand(SerializableFinder finder, this.times, {Duration? timeout})

packages/flutter_driver/test/src/real_tests/stubs/stub_command_extension.dart

@@ -3,7 +3,7 @@
 // found in the LICENSE file.
 
 import 'package:flutter_driver/driver_extension.dart';
-import 'package:flutter_driver/flutter_driver.dart';
+import 'package:flutter_driver/flutter_driver.dart'; // ignore: import_of_legacy_library_into_null_safe
 import 'package:flutter_driver/src/common/message.dart';
 import 'package:flutter_test/flutter_test.dart';
 

packages/flutter_driver/test/src/real_tests/stubs/stub_finder.dart

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_driver/flutter_driver.dart';
+import 'package:flutter_driver/flutter_driver.dart'; // ignore: import_of_legacy_library_into_null_safe
 
 class StubFinder extends SerializableFinder {
   StubFinder(this.keyString);