`flutter create --template=plugin` now include flutter unit test (#26456)

packages/flutter_tools/templates/plugin/pubspec.yaml.tmpl

@@ -11,6 +11,10 @@ dependencies:
   flutter:
     sdk: flutter
 
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
 # For information on the generic Dart part of this file, see the
 # following page: https://www.dartlang.org/tools/pub/pubspec
 

packages/flutter_tools/templates/plugin/test/projectName_test.dart.tmpl

+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:{{projectName}}/{{projectName}}.dart';
+
+void main() {
+  const MethodChannel channel = MethodChannel('{{projectName}}');
+
+  setUp(() {
+    channel.setMockMethodCallHandler((MethodCall methodCall) async {
+      return '42';
+    });
+  });
+
+  tearDown(() {
+    channel.setMockMethodCallHandler(null);
+  });
+
+  test('getPlatformVersion', () async {
+    expect(await {{pluginDartClass}}.platformVersion, '42');
+  });
+}