Used to annotate an instance member (method, getter, setter, operator, or
field) m. Indicates that every invocation of a member that overrides m
must also invoke m. In addition, every method that overrides m is
implicitly annotated with this same annotation.
Used to annotate an instance member in a class or mixin which is meant to
be visible only within the declaring library, and to other instance members
of the class or mixin, and their subtypes.
Used to annotate an instance member that was made public so that it could be
overridden but that is not intended to be referenced from outside the
defining library.