[plugin_platform_interface] Test that `PlatformInterface` and `MockPlatformInterfaceMixinTest` have no instance methods (#4659)

packages/plugin_platform_interface/CHANGELOG.md

diff --git a/packages/plugin_platform_interface/CHANGELOG.md b/packages/plugin_platform_interface/CHANGELOG.md
index 4f49842..72229cb 100644
--- a/packages/plugin_platform_interface/CHANGELOG.md
+++ b/packages/plugin_platform_interface/CHANGELOG.md
@@ -1,3 +1,7 @@
+## NEXT
+
+* Adds additional tests for `PlatformInterface` and `MockPlatformInterfaceMixin`.
+
 ## 2.1.2
 
 * Updates README to demonstrate `verify` rather than `verifyToken`, and to note

packages/plugin_platform_interface/test/plugin_platform_interface_test.dart

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 d36f21c..9e1ddc0 100644
--- a/packages/plugin_platform_interface/test/plugin_platform_interface_test.dart
+++ b/packages/plugin_platform_interface/test/plugin_platform_interface_test.dart
@@ -78,6 +78,14 @@
   ImplementsConstVerifyTokenPluginPlatform() : super(token: const Object());
 }
 
+// Ensures that `PlatformInterface` has no instance methods. Adding an
+// instance method is discouraged and may be a breaking change if it
+// conflicts with instance methods in subclasses.
+class StaticMethodsOnlyPlatformInterfaceTest implements PlatformInterface {}
+
+class StaticMethodsOnlyMockPlatformInterfaceMixinTest
+    implements MockPlatformInterfaceMixin {}
+
 void main() {
   group('`verify`', () {
     test('prevents implementation with `implements`', () {