Return a list containing all of the mixins that are applied to the class being extended in order to derive the superclass of this class.
ClassElement and EnumElement can have mixins.
MixinElement cannot have mixins, so the empty list is 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.
List<InterfaceType> get mixins;