* Handle simple placeholders in plurals

* Handle Date plural placeholders

* Improve variable names

* Turn assert into exceptions, add tests

This reverts commit c235998a.

* Implement date parsing for the gen_l10n tool

* 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

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* Allow plural localized strings to not specify every case

* Canonicalize locale string for gen_l10n.dart script

* Refresh stocks example to use canonicalized locale

* Sort locales and method/properties/getters alphabetically

* Refresh stocks example to show proper sorting

* Sort Localization generation output

- When creating locales that were assumed from existing locales, the
file that is used is non-deterministic. This adds a sort to the iterator
to ensure that the same existing locale is used to generate the assumed
locale
- When generating material, cupertino and date localizations, the generated getters were not sorted. This
introduces an alphabetic sort

* remove main.dart.snapshot

* Add check for placeholders being an empty map

* Remove unnecessary properties from en_ES.arb in the stocks example

* Use getter instead of methods in the stocks example

* Fixed "annotating types for function expression parameters" lint issue from generated localizations delegate code

* Use raw string for l10n description

* Stock app refresh

* Add pubspec.yaml comment

* Add --help flag

* Use l10n tool to generate stocks i18n files

* Rename *_en_EN.arb to *_en_US.arb

* Fix l10n script

*Starting with arb files in lib/l10n, the tool creates the following by default:
1. An AppLocalizations class containing:
- an `AppLocalizationsDelegate`
- a `supportedLocales` property that returns a list of all supported locales.
- a `localizationsDelegate` property that returns a list containing this delegate along with `GlobalMaterialLocalizations.delegate`, `GlobalCupertinoLocalizations.delegate`, and `GlobalWidgetsLocalizations.delegate`.
2. One Dart `message_<locale>.dart` file for each arb file, as well as a `messages_all.dart` file that performs the locale message lookup. The tool infers the locale of each arb file from the @@Locale property or the arb file's name.

https://github.com/dart-lang/sdk/issues/36900

Corects a bnuch of typeos throuhgout teh Fluter codebsae.

Made use of the `misspell` tool:
https://github.com/client9/misspell

Cupertino localization step 8: create a gen_cupertino_localizations and generate one for cupertino english and french (#29824)

Cupertino localization step 7: modularize material specific things out of gen_localizations.dart (#29822)

Cupertino localization step 4: let generated date localization combine material and cupertino locales (#29650)

Cupertino localization step 3: in-place move some material tools around to make room for cupertino (#29644)