pkg/unittest fix deprecation message for mock

pkg/unittest/lib/mock.dart

Index: pkg/unittest/lib/mock.dart
diff --git a/pkg/unittest/lib/mock.dart b/pkg/unittest/lib/mock.dart
index dac509627fb9e064f71cc0921e73f533991439c0..18b6537cf2188f630517a8e37ff170650817ffd2 100644
--- a/pkg/unittest/lib/mock.dart
+++ b/pkg/unittest/lib/mock.dart
@@ -2,10 +2,10 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-/// `unittest.mock` has been moved to the `matcher` package.
+/// `unittest.mock` has been moved to the `mock` package.
 ///
-/// Add `matcher` to your `pubspec.yaml` file and import it via
-/// `import 'package:matcher/mirror_matchers.dart';`
+/// Add `mock` to your `pubspec.yaml` file and import it via
+/// `import 'package:mock/mock.dart';`
 @deprecated
 library unittest.mock;