unsubscribe method
Removes a handler previously added to this Event.
Returns true
if handler was in list, false
otherwise.
This method has no effect if the handler is not in the list.
Implementation
bool unsubscribe(String key) {
if (!_handlers.containsKey(key)) {
print('Key ' + key + " is not subscribed.");
return false;
}
_handlers.remove(key);
return true;
}