The null-aware operators proposal has been accepted by the DEP committee and is in the Dart language spec. Implementation should be underway, and it doesn't need to be behind any sort of flag.

This is the main tracking issue. Individual issues are below:

• VM: VM: Implementing null-aware operators #23455, first released in 1.12.0-dev.0.0
• support Class?.x in the VM: Support Class?.member in the VM #23794
• implement behind a flag in Analyzer: Analyzer: implementing null-aware operators #23456, first released in 1.10.0
• don't emit an error for Class?.x in Analyzer: Confusing error message for "C?.x", where C is a class name #23464
• enable by default in Analyzer: Remove flag for null-aware operators in the Analyzer #23793
• implement behind a flag in dart2js: dart2js: implementing null-aware operators #23457, first released in 1.11.0
• support Class?.x in dart2js: Support Class?.member in dart2js #23795
• enable by default in dart2js: remove flag for dart2js null aware operators #23791, to be released in 1.12.0-dev.3.0
• Documentation (Documenting null-aware operators dart-archive/www.dartlang.org#1364)

Edited by @nex3 to match other similar issues