Add Dart 2.10 annotations to the web engine sources

[jason-simmons]

No description provided.

[jonahwilliams]

LGTM

[nturgut]

I'm not sure why we needed to add these versions instead of changing the file therefore I have question :)

Do we need to also update the unit test files: https://github.com/flutter/engine/blob/master/lib/web_ui/test/text_editing_test.dart#L5

And the test harness files: https://github.com/flutter/engine/blob/master/lib/web_ui/dev/felt.dart#L5

[jonahwilliams]

The unit tests do not need to be updated because they have specifically opted out of nnbd, that is fine. The issue is that the increase in dart version caused the previously opted-in libraries to act as if they were opted out

[nturgut]

The unit tests do not need to be updated because they have specifically opted out of nnbd, that is fine. The issue is that the increase in dart version caused the previously opted-in libraries to act as if they were opted out

Thanks!