Ok, now I can reproduce the error. The reason why it fails in the Arduino Web Editor is because it uses a brand new sketch preprocessing tool named arduino-preprocessor that is still in beta development stage and has some bugs. That tool is not used in any of the release versions of the Arduino IDE. It is used in the beta build of the Arduino IDE and the same error occurs when I compile that program with the beta IDE build.
I don't know why they decided it would be a good idea to make all the users of the Arduino Web Editor unknowing beta testers.