[gen_l10n] Add use-explicit-plural-values option - #194816

flutter gen-l10n currently maps the ICU explicit plural values =0, =1 and =2 to the CLDR plural categories zero, one and two. In many locales Intl.pluralLogic selects those categories for more than the exact number, so a message such as

{count, plural, =1{exactly one} other{{count} items}}

shows "exactly one" for 21, 31 and 101 in Ukrainian, and for 0 in French. In ICU and CLDR, an explicit value matches only that exact number and takes precedence over the plural categories. Because of the mapping, =1 and one also cannot be used in the same message (one of them is dropped with an "overridden by a later plural part" warning), and explicit values other than 0, 1 and 2 are rejected.

This PR adds an opt-in use-explicit-plural-values option (l10n.yaml and --use-explicit-plural-values). When it is enabled, explicit values are generated as exact checks in front of the Intl.pluralLogic call, and only the category cases are passed to Intl.pluralLogic:

String _temp0 = count == 1
    ? 'exactly one'
    : intl.Intl.pluralLogic(
        count,
        locale: localeName,
        other: '$count items',
      );

With the option enabled:

• Any non-negative integer can be used as an explicit value, for example =3 or =10.
• An explicit value and the category of the same name, such as =1 and one, can be used in the same message without a warning.
• Category cases behave exactly as before, including the exact-number handling Intl.pluralLogic already applies to zero, one and two.

The option defaults to false, and the generated code is unchanged when it is off. As discussed in #84291, changing the default would be a breaking change, so that is left for a follow-up.

The generated code uses conditional expressions rather than a switch expression so that it still compiles in packages that use older language versions.

Fixes #84291
Related: #133276 (in Latvian, =0 is currently also used for 10-20 because it is mapped to the zero category; this no longer happens when the option is enabled)

Website documentation: flutter/website#13993

Tests

• dart test test/general.shard/generate_localizations_test.dart test/general.shard/build_system/targets/localizations_test.dart test/commands.shard/hermetic/generate_localizations_test.dart test/general.shard/message_parser_test.dart test/integration.shard/gen_l10n_test.dart
• dart analyze --fatal-infos and dart format --output=none --set-exit-if-changed on the changed files
• Checked manually in a sample app with en, fr and uk ARB files. With the option enabled, =1 is used only for 1 in Ukrainian (21, 31 and 101 fall through to the category cases) and =0 is used for 0 in French. With the option disabled, the behavior is unchanged.

AI assistance disclosure

AI tools (Cursor) assisted with the implementation, tests and verification.

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