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

pkg/unittest/test/teardown_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.teardown_test;
-
-import 'package:unittest/unittest.dart';
-
-import 'package:metatest/metatest.dart';
-
-void main() => initTests(_test);
-
-void _test(message) {
-  initMetatest(message);
-
-  expectTestsPass('teardown test', () {
-    var tornDown = false;
-
-    group('a', () {
-      tearDown(() {
-        tornDown = true;
-      });
-      test('test', () {});
-    });
-
-    test('expect teardown', () {
-      expect(tornDown, isTrue);
-    });
-  });
-}