* Add preferred-supported-locales option to gen_l10n.dart executable

This reverts commit c235998a.

* Add tests for sorting arbPathString, supportedLocales and supportedLanguageCodes alphabetically

* Plural arbPathStrings

* Factor out singleZhMessageArbFileString

* Make translations more accurate and generic

* Add tests to gen_l10n.dart tool

* Separate out LocalizationsGenerator class to improve testability of code

* Add testing dependencies to dev/tools

* Integrate dev/tools testing to flutter CI

* Restructure dev/tools folder for testing

* Fix license headers