deleteDot method
Tombstone the element dot (position-based delete). Returns the block
start, or null if the element is absent.
Implementation
Dot? deleteDot(Dot dot) {
final loc = _locate(dot);
if (loc == null) return null;
if (!loc.$1.deleted.contains(loc.$2)) {
loc.$1.deleted.add(loc.$2);
_visibleCache = null;
}
return loc.$1.start;
}