keyOf static method
The key that produced these exact words, or null when they came from somewhere else.
Identity, so two keys reading Cancel stay apart, and a literal 'Cancel'
written in the UI is correctly not attributed. It fails closed: anything
the app built after the lookup — an interpolation, a toUpperCase() — is
unresolved rather than wrong.
Implementation
static TranslationKey? keyOf(String rendered) => _keys[rendered];