[plugin_platform_interface] Switch mixin to mixin class (flutter#5420)

The test mixin is currently declared as a `class`, which causes problems for projects with a minimum Dart version of 3.0. This switches it to `mixin class`; ideally it would just be a `mixin`, but in theory that could break something, so since this is such a core package in the ecosystem we want to be as conservative as possible. (While the breaking change would only in theory happen to test code, it's possible that someone ignored the `visibleForTesting`, and we have an easy way to avoid the possibility entirely.)

Fixes flutter/flutter#123241

packages/plugin_platform_interface/CHANGELOG.md

@@ -1,3 +1,9 @@
+## 2.1.7
+
+* Changes `MockPlatformInterfaceMixin` to a `mixin class` for better
+  compatibility with projects that have a minumum Dart SDK version of 3.0.
+* Updates minimum supported SDK version to Dart 3.0.
+
 ## 2.1.6
 
 * Adds pub topics to package metadata.

packages/plugin_platform_interface/lib/plugin_platform_interface.dart

@@ -126,4 +126,4 @@ abstract class PlatformInterface {
 ///    implements UrlLauncherPlatform {}
 /// ```
 @visibleForTesting
-abstract class MockPlatformInterfaceMixin implements PlatformInterface {}
+abstract mixin class MockPlatformInterfaceMixin implements PlatformInterface {}

packages/plugin_platform_interface/pubspec.yaml

@@ -15,10 +15,10 @@ issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+
 # be done when absolutely necessary and after the ecosystem has already migrated to 2.X.Y version
 # that is forward compatible with 3.0.0 (ideally the ecosystem have migrated to depend on:
 # `plugin_platform_interface: >=2.X.Y <4.0.0`).
-version: 2.1.6
+version: 2.1.7
 
 environment:
-  sdk: ">=2.19.0 <4.0.0"
+  sdk: ">=3.0.0 <4.0.0"
 
 dependencies:
   meta: ^1.3.0