* [flutter_tools] integration l10n tool

* add runtime skip to build system

* Update build_system.dart

* add links to issues, comments

* Update packages/flutter_tools/test/general.shard/build_system/build_system_test.dart
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>

* Update packages/flutter_tools/lib/src/build_system/targets/localizations.dart
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>

* Update localizations.dart

* switch to gen_l10n localizations

* fix tests
Co-authored-by: Shi-Hao Hong <shihaohong@google.com>