anchor.diff:        break;
       }
       final testCase = _tests[_currentTest];
  -    var f = guardAsync(testCase.run, null, _currentTest);
  +    var f = _guardAsync(testCase.run, null, _currentTest);
       if (f != null) {
         f.whenComplete(() {
           _nextTestCase(); // Schedule the next test.
  
positive.code: 
    invoke([arg0 = sentinel, arg1 = sentinel, arg2 = sentinel,
            arg3 = sentinel, arg4 = sentinel]) {
      return guardAsync(() {
        if (!shouldCallBack()) {
          return;
        } else if (arg0 == sentinel) {
  
negative.code:   }
  
    invoke0() {
      return _guardAsync(
          () {
            if (shouldCallBack()) {
              return callback();
  