true if this class can validly be used as a mixin when defining
another class. The behavior of this method is defined by the Dart Language
Specification in section 9:
It is a compile-time error if a declared or derived mixin refers to super. It is a compile-time error if a declared or derived mixin explicitly declares a constructor. It is a compile-time error if a mixin is derived from a class whose superclass is not Object.
bool get isValidMixin;