Can't use mixins with classes that have named arguments

[Hixie]

In Flutter we use named arguments heavily. The fact that we can't use mixins when the superclass has named arguments is a major problem that keeps getting in our way.

It's not clear why named arguments would be a problem. Just forward the constructor "super" call past the mixin.

[lrhn]

I assume it's the superclass constructors that can't have named parameters?

It seems like an oversight that only constructors with no optional parameters (positional or named) are forwarded. I can't see a practical problem preventing it.

[Hixie]

class A { A({ bool hello }); }
class B { }
class C extends A with B { }

=> This mixin application is invalid because all of the constructors in the base class 'A' have optional parameters.

[floitschG]

Duplicate of
#19226
#17610

and ultimately
#15101

Also dart2js supports it (contrary to the spec, but showing that implementation wise it's feasible), but the VM doesn't.

[Hixie]

See #31543 for the current status of this bug.