[flutter_tools] remove most globals from asset system and remove Cache manipulation in unit tests (#70011)

This reverts commit ff953e4f.

Fixes #69416
Fixes #70121

Necessary to work around #70013 (this looks like it will be fixed, but should protect us in the future)

Restores some notion of skipping pub, with differences from the prior implementation:

Off by default, and only used by the flutter command automatic pub get
A single check parameter. Uses the framework version cached per-project instead of a timestamp of the tool, to avoid cases where downloading a prebuilt framework would cause this check to no longer work.

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

* Added Dartpad sample for Table widget

* Changed template to stateless_widget_scaffold

* Removed trailing character

This reverts commit 6e5845f0.

* [null-safety] migrate flutter gallery to null safety

* Fix crash when widgetspan does not produce a semantics node in render paragraph

* fix comments

* fixed corner case, added test, refactored semantics widget

* addressing comment

* [integration_test] Add a setter for defaultTestTimeout

* type annotation

* More type annotations

* Fix doc

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.

* Ignore import_of_legacy_library_into_null_safe instead of un-migrating.

This will unblock the roll of the next Dart SDK

This reverts commit 7ba775a9.

Instead of loading the use-material-design asset data from a yaml file in the repo, leave it in dart code and simplify. Remove some of the globals, but not enough to update to testWithoutContext