Re-apply "Remove unittest and matcher from the repo."

pkg/unittest/test/group_name_test.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
-// 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.
-
-library unittest.group_name_test;
-
-import 'package:unittest/unittest.dart';
-
-import 'package:metatest/metatest.dart';
-
-void main() => initTests(_test);
-
-void _test(message) {
-  initMetatest(message);
-
-  expectTestResults('group name test', () {
-    group('a', () {
-      test('a', () {});
-      group('b', () {
-        test('b', () {});
-      });
-    });
-  }, [{
-    'description': 'a a'
-  }, {
-    'description': 'a b b'
-  }]);
-}