unregister<TEvent> method
Unregisters the given handler.
Implementation
void unregister<TEvent>(EventHandler<TEvent> handler) {
final handlers = _getHandlersFor(TEvent);
assert(
handlers.contains(handler),
'unregister<$TEvent> was called for a handler that was never registered',
);
// When the store is being modified, create a new copy.
final update = handlers.toSet()..remove(handler);
_handlers[TEvent] = update;
}