[plugin_platform_interface] Use Mockito nnbd (#3437)

diff --git a/packages/plugin_platform_interface/CHANGELOG.md b/packages/plugin_platform_interface/CHANGELOG.md
index 7df1834..96533f0 100644
--- a/packages/plugin_platform_interface/CHANGELOG.md
+++ b/packages/plugin_platform_interface/CHANGELOG.md
@@ -1,3 +1,7 @@
+## 1.1.0-nullsafety.2
+
+* Use Mockito null safe.
+
 ## 1.1.0-nullsafety.1
 
 * Bump Dart SDK to support null safety.
diff --git a/packages/plugin_platform_interface/lib/plugin_platform_interface.dart b/packages/plugin_platform_interface/lib/plugin_platform_interface.dart
index cd87b04..6e425a1 100644
--- a/packages/plugin_platform_interface/lib/plugin_platform_interface.dart
+++ b/packages/plugin_platform_interface/lib/plugin_platform_interface.dart
@@ -43,7 +43,7 @@
   /// Pass a private, class-specific `const Object()` as the `token`.
   PlatformInterface({required Object token}) : _instanceToken = token;
 
-  final Object _instanceToken;
+  final Object? _instanceToken;
 
   /// Ensures that the platform instance has a token that matches the
   /// provided token and throws [AssertionError] if not.
diff --git a/packages/plugin_platform_interface/pubspec.yaml b/packages/plugin_platform_interface/pubspec.yaml
index 05fc918..084e577 100644
--- a/packages/plugin_platform_interface/pubspec.yaml
+++ b/packages/plugin_platform_interface/pubspec.yaml
@@ -12,7 +12,7 @@
 # be done when absolutely necessary and after the ecosystem has already migrated to 1.X.Y version
 # that is forward compatible with 2.0.0 (ideally the ecosystem have migrated to depend on:
 # `plugin_platform_interface: >=1.X.Y <3.0.0`).
-version: 1.1.0-nullsafety.1
+version: 1.1.0-nullsafety.2
 
 repository: https://github.com/flutter/plugins/tree/master/packages/plugin_platform_interface
 
@@ -23,6 +23,6 @@
   meta: ^1.3.0-nullsafety.3
 
 dev_dependencies:
-  mockito: ^4.1.1
+  mockito: ^5.0.0-nullsafety.2
   test: ^1.10.0-nullsafety.1
   pedantic: ^1.10.0-nullsafety.1
diff --git a/packages/plugin_platform_interface/test/plugin_platform_interface_test.dart b/packages/plugin_platform_interface/test/plugin_platform_interface_test.dart
index b07dd4d..0488c20 100644
--- a/packages/plugin_platform_interface/test/plugin_platform_interface_test.dart
+++ b/packages/plugin_platform_interface/test/plugin_platform_interface_test.dart
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// TODO(egarciad): Remove once Mockito is migrated to null safety.
-// @dart = 2.9
 import 'package:mockito/mockito.dart';
 import 'package:test/test.dart';