Updates all null safe dependencies to versions that allow 2.10 stable and 2.11 dev releases.

Also updates flutter_goldens and flutter_goldens_client to allow 2.11 dev.

[versions] update to latest source span and roll engine to  4b8477d11573d233e6791204191c0090f733b05d (#66136)

Update to latest dwds, file-nullsafety, and process-nullsafety for #62886

Updates all the dependencies used in the most basic test and flutter_test isolates.

This reverts commit c46aa360.

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

Roll packages, update XML deprecations.

Fixes #59718

Update all versions, specifically dwds which has a hot restart fix

* Bump flutter_gallery_assets version to ^0.2.0.

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

* Add comment.

* Remove duplicate key.

Prefer using `.of()` (which is statically type-checked) over
`.from()` (which is dynamically type-checked) when possible.

This moves the app template more toward being a more generic starting
point for any Flutter application, eliminating some hard-code
assumptions about there being a single window/engine pair that is
directly bound to the life of the application:
- Moves the runloop into its own class, making it capable of servicing
  any number of engine instances.
- Moves the logic for setting up a window containing only a Flutter view
  into a window subclass for ease of re-use.
- Makes quit-on-window-close an optional property. (Long term this
  should be even more generic, like a quit-when-last-window-closes
  option, but this is a short-term improvement that removes the binding
  between the runloop and the window).
- Allows for multiple instances of Win32Window to exist without issues
  relating to the window class registration.

Since there are getting to be a non-trivial number of files associated
with the runner, this moves the source into a runner/ directory, as is
already done on some other platforms.

Note that creating multiple Flutter windows at the same time still
doesn't work correctly even with this change, but this addresses some of
the known issues, and makes it easier to test in the future (e.g., for
debugging engine-level issues with multiple instances).

Fixes #45397

Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.

This is required to roll forward package:vm_service to 3.0.0 and is
blocking a DevTools release.