updateSnapshot function
The generator's merge: every entry of committed as it is, plus the
entries of current it lacks. Never rewrites an existing entry, so a
changed geometry shows as a failing conformance test, never as a quietly
regenerated file.
Implementation
SplayTreeMap<String, String> updateSnapshot(
Map<String, String> committed, Map<String, String> current) =>
SplayTreeMap.of({...current, ...committed});