Force opt-in flutter test platform to null safety for repo testing

Re-arrange the implementation of the devtools launcher so that google3 is not required to depend on any devtools packages. Also renames the build_runner folders to isolated to better clarify their intention.

Allow configuring the flutter_manifest to support a synthetic package, this is done through flutter: generate: true.

When running pub get, insert a flutter_gen entry into the packages if it does not already exist. This points to .dart_tool/flutter_gen, which can be updated to contain the generated intl sources (But doesn't currently)

Adds an integration test that verifies this code can be run and imported when enabled.

Part of #60914

* Reland: [flutter_tools] File system utilities

* Remove unused imports

This reverts commit 03496606.

Revert "flutter_tools: Remove unused //ignore comments; change flutter-specific ones to "ignore_for_flutter" (#45917)" (#46999)

This reverts commit f2859201.

It broke forbidden_imports_test, which uses a custom ignore directive.

flutter_tools: Remove unused //ignore comments; change flutter-specific ones to "ignore_for_flutter" (#45917)

* 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

No code changes, just comments: I spell-checked all the comments in the repo.

* WIP on web plugin registry

* WIP on registering plugins

* WIP on web plugin registration

* Only generate `package:flutter_web_plugins` imports if plugins are
defined

* Add parsing test

* Add documentation

* Fix analyzer warnings

* add license headers

* Add tests for package:flutter_web_plugins

* Run `flutter update-packages --force-upgrade`

* Fix analyzer errors

* Fix analyzer error in test

* Update copyright and remove flutter SDK constraints

* Enable tests since engine has rolled

* add flutter_web_plugins tests to bots

* Create an empty .packages file for WebFs test

* fix multiroot builds

* remove shout

* add basic test

* Update web_compilation_delegate.dart

* Address review feedback

* Update multiroot_asset_reader_test.dart

* Update multiroot_asset_reader_test.dart

* remove unpassable test

* fix for windows

* facepalm

This reverts https://github.com/flutter/flutter/pull/33525

This reverts commit 52ae9968.