debugAssertVisibleSubtreeSizeConsistency method
Debug-only forwarder preserved for the existing fuzz test surface
(test/sliver_tree/visible_subtree_size_invariant_fuzz_test.dart,
the four purge_* tests, etc.). After Plan B the assertion lives
inside VisibleOrderBuffer.debugAssertSubtreeSizeConsistent.
Implementation
@visibleForTesting
void debugAssertVisibleSubtreeSizeConsistency() =>
_order.debugAssertSubtreeSizeConsistent();