anchor.diff:  
     Future<WaitForResult> _waitFor(Command command) async {
       final WaitFor waitForCommand = command;
  -    if ((await _waitForElement(_createFinder(waitForCommand.finder))).evaluate().isNotEmpty)
  -      return new WaitForResult();
  -    else
  -      return null;
  +    await _waitForElement(_createFinder(waitForCommand.finder));
  +    return new WaitForResult();
     }
   
     Future<WaitForAbsentResult> _waitForAbsent(Command command) async {
  
positive.code:     if (_frameSync)
        await _waitUntilFrame(() => SchedulerBinding.instance.transientCallbackCount == 0);
  
      await _waitUntilFrame(() => !finder.precache());
  
      if (_frameSync)
        await _waitUntilFrame(() => SchedulerBinding.instance.transientCallbackCount == 0);
  
negative.code:     if (_frameSync)
        await _waitUntilFrame(() => SchedulerBinding.instance.transientCallbackCount == 0);
  
      await _waitUntilFrame(() => finder.evaluate().isNotEmpty);
  
      if (_frameSync)
        await _waitUntilFrame(() => SchedulerBinding.instance.transientCallbackCount == 0);
  