Uint64List should acknowledge that int is signed

[Hixie]

The Uint64 list docs (https://api.flutter.dev/flutter/dart-typed_data/Uint64List-class.html) don't make any mention of the fact that you can't actually get an unsigned int out of a Uint64, since Dart's integers are signed 64 bit.

(It's not really clear to me that we should keep Uint64List at all, actually. As far as I can tell it's not really usable now that we don't have unbounded integers.)

[Hixie]

See also #35225

[lrhn]

We could consider removing the class entirely (it doesn't exist on the web, so it's already half-way not there).
If we introduce a typedef Uint64List = Int64List;, no-one should be able to tell the difference.