Return the superclass of this class, or
null if either the class
represents the class 'Object' or if the class represents a mixin
declaration. All other classes will have a non-
null superclass. If the
superclass was not explicitly declared then the implicit superclass
'Object' will be returned.
Note: Because the element model represents the state of the code, it is possible for it to be semantically invalid. In particular, it is not safe to assume that the inheritance structure of a class does not contain a cycle. Clients that traverse the inheritance structure must explicitly guard against infinite loops.
InterfaceType get supertype;