Field-less mixins should be allowed for const constructors.

[johnniwinther]

Const constructors should be allowed if mixed-in classes have no fields:

class Super {}
class Mixin {}
class Class extends Super with Mixin {
  const Class(); // This is currently not allowed.
}

[lrhn]

See also #9745

General constructor support isn't forthcoming, so maybe just allowing constructor forwarding to be const in the case where the superclass constructor is const and the mixin has no instance variables, will go a long way.

[eernstg]

Closing: As of 63c6851, constructor forwarding does just that. Created #32223 to put this into the implementation pipeline.