anchor.diff:   * in that it removes the tests completely.
    */
   void filterTests(testFilter) {
  -  _requireNotRunning();
  -  Function filterFunction;
  +  var filterFunction;
     if (testFilter is String) {
       RegExp re = new RegExp(testFilter);
       filterFunction = (t) => re.hasMatch(t.description);
  
positive.code:             return callback(arg1);
            }
          },
          after, testCase);
    }
  
    invoke2(arg1, arg2) {
  
negative.code:             return callback(arg1, arg2);
            }
          },
          after, testNum);
    }
  }
  
  /**
   * Indicate that [callback] is expected to be called a [count] number of times
   * (by default 1). The unittest framework will wait for the callback to run the
   * specified [count] times before it continues with the following test.  Using
   * [_expectAsync] will also ensure that errors that occur within [callback] are
   * tracked and reported. [callback] should take between 0 and 4 positional
   * arguments (named arguments are not supported here). [id] can be used
   * to provide more descriptive error messages if the callback is called more
   * often than expected.
   */
  Function _expectAsync(Function callback,
                       {int count: 1, int max: 0, String id}) {
    return new _SpreadArgsHelper(callback, count, max, null, id).invoke;
  }
  
  /**
  