I began to debug, and fixed several #include errors, but finally quit in exasperation. It seems that several Adafruit_ASFcore header files are not compatible with the Arduino web IDE.
After the compilation fails click the copy icon at the top right corner of the black console window (you might have to drag the divider at the bottom of the editor window to make the console window show). Then paste it in a reply here using code tags.
I confirm the issue. Note that it also occurs with the beta build of the Arduino IDE. This indicates the problem is probably with arduino-preprocessor. I have reported the issue, with minimum steps to reproduce the problem, here:
For some reason they decided to turn all Arduino Web Editor users into beta testers without their knowledge, thus you will be more likely to encounter this sort of unpleasant surprise using the Arduino Web Editor. Beta testers are always welcome and that's easy enough by downloading the hourly build or beta build from:
but to subject Arduino Web Editor users, who may be struggling with the basics of programming and electronics, who may have resorted to the Arduino Web Editor because they thought a "cloud based" IDE would be easier than trying to install the standard IDE (false because you still need to install the plugin for the Arduino Web Editor), to use a not fully tested potentially unstable application is insane.