Specify primary constructors

[eernstg]

This PR adds a proposed feature specification for primary constructors, applicable to normal classes as well as inline classes.

This PR has been merged. It added this feature specification.

[eernstg]

A bunch of different declarations can be explored by means of scripts/show_primary_constructors.dart. Try

> cd scripts
> dart show_primary_constructors.dart --help
...
> dart show_primary_constructors.dart --show-normal --show-struct class.json
...

to see some help and a single example. Try using --show-keyword to show examples in the style where the primary constructor uses new, const, new.named, const.named and occurs after superinterfaces. E.g., to compare the class const Point._(...) implements C; and the class Point implements C const._(...); style with named constructors:

> dart show_primary_constructors.dart -sk *named*.json

[eernstg]

Review response.

[mit-mit]

Very excited about this potential feature!

[eernstg]

Thanks, @lrhn, very useful feedback! I've closed a bunch of threads that I thought were uncontroversial, and left others open.

It is indeed tricky to specify anything in terms of a desugaring step. Thinking about it. ;-)

[eernstg]

Many smaller improvements introduced in response to review comments, PTAL.

[eernstg]

The language team agreed to land this proposal at this time. It is in the 'working' directory, which implies that it is a proposal, not a design which has received a final acceptance.