Picks up Dart SDK roll to 3ee0a4284203ebc6991c78054583a7c02dc8faf9

This is the first engine role with libwebp, so we expect an increase in
flutter app sizes.
I'll be monitoring the app size benchmarks after this lands.

This reverts commit 5939ff45.

I'm not really sure how to test this without a reference test, since
eventually we want to move this to a font feature and thus the obvious
test (looking for the Transform widget) isn't really valid.

* enable lint prefer_asserts_in_initializer_lists

* enable --assert-initializer

* Support accessibility labels and hints

* more tests

* ++

* review comments

* fix merge

* test fix

* Revert "Fix error message to refer to actual ClipRect widget instead of non-existing RectClip (#12632)"

This reverts commit 1573d3b8.

* Revert "Roll engine to 5003703330f280325fa73080bb23b1ab7008b213 (#12629)"

This reverts commit 81a358b7.

* Fix restart flow for preview-dart-2 mode.

Restart in preview-dart-2 needs to use kernel file and it has to be complete, rather than incremental kernel file.

* Add curly braces

* Do full compile on restart

* Roll engine to pick up changes to hot reload for preview-dart-2