Flow analysis - keep non-nullability on join

[scheglov]

Here ancestor has initially type Element?, so cannot be promoted to either ClassElement or ExtensionElement. But it cannot be null if any of them is true.

    var ancestor = enclosingElement;
    if (ancestor is ClassElement || ancestor is ExtensionElement) {
      if (ancestor.hasDoNotStore) {
        return true;
      }
    }