Move from `List<Int>` to `Uint8List`

[truongsinh]

Ref: https://groups.google.com/a/dartlang.org/forum/#!topic/misc/JpNkRRLI9_w, google/protobuf.dart#219

More and more packages are explicitly switching to Uint8List.

[daegalus]

Unnecessary change. Nothing I use returns Uint8list, its easy to convert between, and in the topic you linked, they say as much. If you need Uint8list, just convert it internally. I wont be changing this.