forget method Null safety
Causes the given child to be removed from the list of children to which this object might defer, as if deferTo had never been called for that child.
This should only be called once per child, even if deferTo was called multiple times for that child.
If no children are left in the list, this object will stop deferring to its children. (This is not the same as calling takePriority, since, if this object itself is a ChildBackButtonDispatcher, takePriority would additionally attempt to claim priority from its parent, whereas removing the last child does not.)
void forget(ChildBackButtonDispatcher child) => _children.remove(child);