[image_picker] Do not copy a static field into another static field

packages/image_picker/image_picker/lib/image_picker.dart

@@ -27,7 +27,7 @@ export 'package:image_picker_platform_interface/image_picker_platform_interface.
 class ImagePicker {
   /// The platform interface that drives this plugin
   @visibleForTesting
-  static ImagePickerPlatform platform = ImagePickerPlatform.instance;
+  static ImagePickerPlatform get platform => ImagePickerPlatform.instance;
 
   /// Returns a [File] object pointing to the image that was picked.
   ///

packages/image_picker/image_picker/pubspec.yaml

@@ -25,7 +25,9 @@ dev_dependencies:
     sdk: flutter
   integration_test:
     path: ../../integration_test
+  mockito: ^4.1.3
   pedantic: ^1.8.0
+  plugin_platform_interface: ^1.0.3
 
 environment:
   sdk: ">=2.1.0 <3.0.0"

packages/image_picker/image_picker/test/image_picker_test.dart

@@ -5,6 +5,9 @@
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:image_picker/image_picker.dart';
+import 'package:image_picker_platform_interface/image_picker_platform_interface.dart';
+import 'package:mockito/mockito.dart';
+import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
@@ -26,6 +29,15 @@ void main() {
       log.clear();
     });
 
+    test('ImagePicker platform instance overrides the actual platform used',
+        () {
+      final ImagePickerPlatform savedPlatform = ImagePickerPlatform.instance;
+      final MockPlatform mockPlatform = MockPlatform();
+      ImagePickerPlatform.instance = mockPlatform;
+      expect(ImagePicker.platform, mockPlatform);
+      ImagePickerPlatform.instance = savedPlatform;
+    });
+
     group('#pickImage', () {
       test('passes the image source argument correctly', () async {
         await picker.getImage(source: ImageSource.camera);
@@ -336,3 +348,7 @@ void main() {
     });
   });
 }
+
+class MockPlatform extends Mock
+    with MockPlatformInterfaceMixin
+    implements ImagePickerPlatform {}