anchor.diff:    }
   }
   
  -class _Sentinel {
  -  static final _sentinel = const _Sentinel();
  -
  -  const _Sentinel();
  -}
  -
  -
  -/**
  - * Indicate to the unittest framework that a callback is expected. [callback]
  - * can take any number of arguments between 0 and 4.
  +/**
  + * Indicate to the unittest framework that a 0-argument callback is expected.
    *
    * The framework will wait for the callback to run before it continues with the
    * following test. The callback must excute once and only once. Using [later]
    * will also ensure that errors that occur within the callback are tracked and
    * reported by the unittest framework.
    */
  -// TODO(sigmund): expose this functionality
  -Function _later(Function callback) {
  -  Expect.isTrue(_currentTest < _tests.length);
  -  var testCase = _tests[_currentTest];
  -  testCase.callbacks++;
  -  // We simulate spread arguments using named arguments:
  -  // Note: this works in the vm and dart2js, but not in frog.
  -  return ([arg0 = _Sentinel.value, arg1 = _Sentinel.value,
  -           arg2 = _Sentinel.value, arg3 = _Sentinel.value,
  -           arg4 = _Sentinel.value]) {
  -    _guard(() {
  -      if (arg0 == _Sentinel.value) {
  -        callback();
  -      } else if (arg1 == _Sentinel.value) {
  -        callback(arg0);
  -      } else if (arg2 == _Sentinel.value) {
  -        callback(arg0, arg1);
  -      } else if (arg3 == _Sentinel.value) {
  -        callback(arg0, arg1, arg2);
  -      } else if (arg4 == _Sentinel.value) {
  -        callback(arg0, arg1, arg2, arg3);
  -      } else {
  -        testCase.error(
  -           'unittest lib does not support callbacks with more than 4 arguments',
  -           '');
  -        _state = _UNCAUGHT_ERROR;
  -      }
  -    }, callbackDone);
  -  };
  -}
  -
   // TODO(sigmund): expose this functionality
   Function _later0(Function callback) {
     Expect.isTrue(_currentTest < _tests.length);
  
positive.code: }
  
  // TODO(sigmund): expose this functionality
  Function _later1(Function callback) {
    Expect.isTrue(_currentTest < _tests.length);
    var testCase = _tests[_currentTest];
  
negative.code: }
  
  // TODO(sigmund): expose this functionality
  /** Like [_later0] but expecting a callback with 2 arguments. */
  Function _later2(Function callback) {
    Expect.isTrue(_currentTest < _tests.length);
    var testCase = _tests[_currentTest];
  