pls add syntactic sugars of List and Map

1. add property or getter of selected to List;

2. list?.asMap()?.keys?.map(...) =>list?.asMap().keys.map(...);
when ?. get null , cade after should stop ;

3. {...map} , map should can be null like javascript?

The issue names three proposed Dart syntax changes for List, Map, null-aware access, and spread syntax, but no files, tests, or entry points. Start by reviewing the language specification and design process for these constructs. Done would require an agreed design and corresponding specification changes.

Written by the indexing model from the issue text.