Meta-issue for discussion of optional const and optional new

[eernstg]

This issue is to provide a forum for community discussion and feedback related to the optional const and optional new features.

An informal specification can be found here: optional const, optional new.

cc @munificent @leafpetersen @floitschG @lrhn

[munificent]

See also: #4046, #18241, #20750

[srawlins]

This shipped in Dart 2.0.