packageSubtreeBytes function
Sums the bytes of the package:<package> subtree of an analyze-size
JSON tree: the node's own value (null on pure containers) plus every
descendant's value, exactly as the DevTools size analysis accounts bytes.
Returns null when no such node exists (the app does not import the solution under that pub package name).
Implementation
int? packageSubtreeBytes(dynamic tree, String package) {
if (tree is! Map<String, dynamic>) return null;
final node = _findNode(tree, 'package:$package');
if (node == null) return null;
return _sumNode(node);
}