Adds a new member (e.g., gesture recognizer) to the arena.
Source
GestureArenaEntry add(int pointer, GestureArenaMember member) {
_GestureArena state = _arenas.putIfAbsent(pointer, () => new _GestureArena());
state.add(member);
return new GestureArenaEntry._(this, pointer, member);
}