java.io does have a FileReader class, but I think Code.org created their own FileReader with the toXXXXArray methods. So I assume the code.org created FileReader class will be hanging off org.code somewhere. I tried some guesses (like import org.code.io.FileReader) but no joy.
I wish code.org would document all of the libraries the use the JavaLab documentation.
For reference, this is the java.io.FileReader documentation.
FileReader (Java Platform SE 8 ) (oracle.com)