Migrate core devicelab framework to null safety.

5f445d7c0 Roll Dart SDK from 812bf07cf737 to 40695d910591 (1 revision) (flutter/engine#27306) (#86239)

0a892d9ec Roll Dart SDK from ab8bac9fd542 to 812bf07cf737 (1 revision) (flutter/engine#27300) (#86215)

36271dc62 Roll Dart SDK from 83a559e0a9bd to ab8bac9fd542 (1 revision) (flutter/engine#27298) (#86212)

731b93d52 [image_picker_for_web] Migrate image_picker to package:cross_file (flutter/plugins#4083) (#86207)

8a966ebfd [various] Prepare plugin repo for binding API improvements (flutter/plugins#4148) (#86203)

* [web] move e2e tests from flutter/engine to flutter/flutter
* flutter update-packages --force-upgrade

Fixes a bug on Android Gboard where the handle would disappear when dragged within a word.

2dfb40061 Roll Dart SDK from ae01d698532b to a8c4933bf92e (2 revisions) (flutter/engine#27279) (#86175)

This reverts commit 501474de.

53d1a53ac Roll Dart SDK from eb6ae1c165fc to ae01d698532b (2 revisions) (flutter/engine#27270) (#86151)