navigationIndex method
Index of a source position in navigation, or -1 when absent.
Implementation
int navigationIndex(DiffCommentSide side, DiffCommentLineKey key) =>
_navigationIndices.putIfAbsent(side, () {
final keys = navigation(side);
return {for (var i = 0; i < keys.length; i++) keys[i]: i};
})[key] ??
-1;