Abandon "Value classes" proposal after macros are landing in dart

[MohiuddinM]

Values classes proposal is superseded by macros, and macros are being added to dart.

[mraleph]

Changes to identity behavior and copyWith semantics as described in that proposal can't be implemented through macros. Macros can help generating the boilerplate though.

[munificent]

It's just a proposal, so we don't need to officially "abandon" it since all proposals default to being in a "not going to be shipped" state unless they get accepted. So I don't think there's anything we need to take action on for this issue.