https://forum.pjrc.com/threads/63842

Sketches that includes String objects did not compile, so simply copy the wstring.cpp from Teensy3 directory.

Likewise sketches that used stream functions like:
Serial1.readBytes would not compile as the Stream  sources were not included.

So  likewise copied over from Teensy3 directory

https://forum.pjrc.com/threads/35688-Something-wrong-with-Adding-a-String-to-itself!?p=111109#post111109