Return a list containing all of the superclass constraints defined for
this class. The list will be empty if this class does not represent a
mixin declaration. If this class does represent a mixin declaration but
the declaration does not have an
on clause, then the list will contain
the type for the class
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.
List<InterfaceType> get superclassConstraints;