Use case

When writing Plugin tests, MockPlatformInterfaceMixin is provided to be able to mock PlatformInterface.
However, MockPlatformInterfaceMixin is defined as a class, which conflicts with linter rule prefer_mixin (https://dart-lang.github.io/linter/lints/prefer_mixin.html)

Proposal

Change MockPlatformInterfaceMixin from class to mixin

mixin MockPlatformInterfaceMixin implements PlatformInterface {}