* Move web integration tool tests to web.shard

Web integration tool tests depend on DDC changes in SDK. This change
moves them to a separate shard and subshard so CI bot configurations
can run them separately.

In particular, with will allow running those tests on dart CI flutter
HHH web bot instead of a non-web one, allowing early detection and easy
classification of issues caused by SDK changes as VM- or Web related.

* Enabled verbose mode for flaky web_tool_tests

* fixed local engine crash with --enable-asserts

* Disable failing tests, fix local engine with asserts

* Removed mac web_tool_tests bot

* Addressed CR comments

* Add disabled mac web_tool_tests and disable win web_tool_tests

This reverts commit 2a5aa294.

Additionally, fix a few NNBD issues.

Fixes https://github.com/flutter/flutter/issues/70408

Address comment in https://github.com/flutter/flutter/pull/66386/files#r522671103

Migrate flutter_localizations to null safety.

This reverts commit a0ec4d67.

Unit tests that cover their translations are also added.

This allows the two videos in the demo to play simultaneously
(matching the behavior from before the recent upgrade to the
video player plugin)

* Move web integration tool tests to web.shard

Web integration tool tests depend on DDC changes in SDK. This change
moves them to a separate shard and subshard so CI bot configurations
can run them separately.

In particular, with will allow running those tests on dart CI flutter
HHH web bot instead of a non-web one, allowing early detection and easy
classification of issues caused by SDK changes as VM- or Web related.

* Enabled verbose mode for flaky web_tool_tests

* Split out the test changes to be commited first

Remove references to `Window`, and switch usages to `PlatformDispatcher` or `SingletonFlutterWindow` (#69617)

* Remove references to dart:ui.Window, and point usages to PlatformDispatcher or SingletonFlutterWindow, as appropriate

* remove new test platform dispatchers

* Amend documentation

This reverts ae0a9cb5 and therefore relands #69629.

Additionally, `flutter update-packages --force-upgrade` has been run to update `pubspec.yaml` to resolve the analyze test failures.

This reverts commit 7ba775a9.