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

pkg/unittest/test/breath_test.dart

-// Copyright (c) 2012, 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.breath_test;
-
-import 'dart:async';
-
-import 'package:unittest/unittest.dart';
-
-void main() {
-  // Test the sync test 'breath' feature of unittest.
-
-  // We use the testStartStopwatch to determine if the 'starve'
-  // test was executed within a small enough time interval from
-  // the first test that we are guaranteed the second test is
-  // running in a microtask. If the second test is running as a
-  // microtask we are guaranteed the timer scheduled in the
-  // first test has not been run yet.
-  var testStartStopwatch = new Stopwatch()..start();
-
-  group('breath', () {
-    var sentinel = 0;
-
-    test('initial', () {
-      Timer.run(() {
-        sentinel = 1;
-      });
-    });
-
-    test('starve', () {
-      // If less than BREATH_INTERVAL time has passed since before
-      // we started the test group then the previous test's timer
-      // has not been run (at least this is what we are testing).
-      if (testStartStopwatch.elapsed.inMilliseconds <= BREATH_INTERVAL) {
-        expect(sentinel, 0);
-      }
-
-      // Next we wait for at least BREATH_INTERVAL to guaranteed the
-      // next (third) test is run using a timer which means it will
-      // run after the timer scheduled in the first test and hence
-      // the sentinel should have been set to 1.
-      var sw = new Stopwatch()..start();
-      while (sw.elapsed.inMilliseconds < BREATH_INTERVAL);
-    });
-
-    test('breathed', () {
-      expect(sentinel, 1);
-    });
-  });
-}