l10n tool improvements, stocks app refresh

[shihaohong]

Description

Adds a help flag, as well as help messages to the gen_l10n.dart tool. This also refreshes the stocks app to reflect the new changes to the tool, while fixing a typo in the regenerate.md tutorial.

~/dev/flutter$ dart ${FLUTTER}/dev/tools/localization/gen_l10n.dart --help
--help                        Print this help message.
--arb-dir                     The directory where all localization files should reside. For example, the template and translated arb files should be located here. Also, the generated output messages Dart files for each locale and the generated localizations classes will be created here.
                              (defaults to "lib/l10n")

--template-arb-file           The template arb file that will be used as the basis for generating the Dart localization and messages files.
                              (defaults to "app_en.arb")

--output-localization-file    The filename for the output localization and localizations delegate classes.
                              (defaults to "app_localizations.dart")

--output-class                The Dart class name to use for the output localization and localizations delegate classes.
                              (defaults to "AppLocalizations")

Issues

Related to #41437

[fluttergithubbot]

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[HansMuller]

LGTM

[HansMuller]

Including "rest of dependencies" twice doesn't look right. Maybe just include the second one.

[HansMuller]

Since developers will likely cut-and-paste this, maybe include a comment. For example:

///   # Internationalization support. 
///   flutter_localizations:
///     sdk: flutter
...

Actually, when there's a gen_l10n doc to point at, the comment should include a link.