The class symbol scala.Nothing is mapped to the class scala.runtime.Nothing$. Similarly,
scala.Null is mapped to scala.runtime.Null$. This is because there exist no class files
for the Nothing / Null. If used for example as a parameter type, we use the runtime classes
in the classfile method signature.
Note that the referenced class symbol may be an implementation class. For example when
compiling a mixed-in method that forwards to the static method in the implementation class,
the class descriptor of the receiver (the implementation class) is obtained by creating the
ClassBType.
