* [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

This reverts commit dd279e41.

Add a --dart-entrypoint-args flag to flutter run to pass through Dart entrypoint arguments on Flutter Desktop (#69607)

Commit https://github.com/flutter/engine/pull/22348 appears to have
caused breakage https://github.com/flutter/flutter/issues/69983
[tool_crash] RPCError.

This reverts commit 9edfa122.