The demo app depends on a generated source file called
l10n/messages_all.dart, which defines all of the
localizable strings used by the app.

Rebuilding l10n/messages_all.dart requires two steps.

With the app’s root directory as the current directory,
generate l10n/intl_messages.arb from lib/main.dart:

    
$ dart run intl_translation:extract_to_arb --output-dir=lib/l10n lib/main.dart
    

    The intl_messages.arb file is a JSON format map with one entry for
each Intl.message() function defined in main.dart. This
file serves as a template for the English and Spanish translations,
intl_en.arb and intl_es.arb.
These translations are created by you, the developer.