[gen_l10n] Flat inheritance model for subclasses leads to incorrect runtime fallbacks for multi-level locales #187948

Description

The gen_l10n tool in Flutter currently generates a flat inheritance model for subclasses (regional and script variants of a language). All subclasses for a given language code extend the base language class directly, rather than forming a hierarchical chain.

This leads to incorrect runtime fallback behavior for multi-level locales (e.g., Language + Script + Region).

Reproduction Steps

Consider the following ARB files in a project:

1. app_zh.arb (Base Simplified Chinese):

   {
     "@@locale": "zh",
     "dry": "干"
   }

2. app_zh_Hant.arb (Traditional Chinese):

   {
     "@@locale": "zh_Hant",
     "dry": "乾"
   }

3. app_zh_Hant_HK.arb (Hong Kong Traditional Chinese - empty, expecting to inherit everything from zh_Hant):

   {
     "@@locale": "zh_Hant_HK"
   }

Expected Behavior:
At runtime, if the app is run in zh_Hant_HK locale, looking up the dry key should return "乾" (inherited from zh_Hant).

Actual Behavior:
The app returns "干" (inherited from zh).

Technical Details

In packages/flutter_tools/lib/src/localizations/gen_l10n.dart, the generator determines the base class for all subclasses using only the languageCode:

final baseClassName = '$className${LocaleInfo.fromString(locale.languageCode).camelCase()}';