This is a reland of commit b2f4cf3e01
Commented out deprecation for now.
Original change's description:
> Add class modifiers to `dart:convert`.
>
> The usual approach:
> Pure interfaces marked `interface`.
> Pure implementation classes marked `final`.
> Base classes marked `base` or nothing, and `mixin class` if reasonable.
> Combined X/XBase/XMixin where possible.
>
> CoreLibraryReviewExempt: Aske is away
> Change-Id: I927f9bd488fb385ff9c17c8fc94920a1f5076347
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/289200
> Reviewed-by: Stephen Adams <sra@google.com>
> Reviewed-by: Slava Egorov <vegorov@google.com>
> Reviewed-by: Nate Bosch <nbosch@google.com>
> Commit-Queue: Lasse Nielsen <lrn@google.com>
CoreLibraryReviewExempt: Approved in original.
Change-Id: I1bc14f99b742567e2634dcfcbc52f332dbcc5364
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/290521
Reviewed-by: Nate Bosch <nbosch@google.com>
Commit-Queue: Lasse Nielsen <lrn@google.com>