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

pkg/unittest/test/late_exception_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.late_exception_test;
-
-import 'dart:async';
-
-import 'package:metatest/metatest.dart';
-import 'package:unittest/unittest.dart';
-
-void main() => initTests(_test);
-
-void _test(message) {
-  initMetatest(message);
-
-  expectTestResults('late exception test', () {
-    var f;
-    test('testOne', () {
-      f = expectAsync(() {});
-      new Future.sync(f);
-    });
-    test('testTwo', () {
-      new Future.sync(expectAsync(() {
-        f();
-      }));
-    });
-  }, [{
-    'description': 'testOne',
-    'message': 'Callback called (2) after test case testOne has already been '
-        'marked as pass.',
-    'result': 'error',
-  }, {
-    'description': 'testTwo',
-    'result': 'pass',
-  }]);
-}