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

pkg/unittest/test/testcases_immutable_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.testcases_immutable;
-
-import 'package:unittest/unittest.dart';
-
-import 'package:metatest/metatest.dart';
-
-void main() => initTests(_test);
-
-void _test(message) {
-  initMetatest(message);
-
-  expectTestsPass('testcases immutable', () {
-    test('test', () {
-      expect(() => testCases.clear(), throwsUnsupportedError);
-      expect(() => testCases.removeLast(), throwsUnsupportedError);
-    });
-  });
-}