state property
The mutable state owned by this branch.
@protected: only this branch and its subclasses reach it (a domain
element upgrades the return type; an actionable element forwards to it).
It is not public API — external layers must not reach into a branch's
State. Tests that need it access it with an
invalid_use_of_protected_member ignore.
Implementation
@protected
State<StatefulSeed> get state => _state;