Allow mixins to be used with const constructors

[nex3]

A mixin cannot explicitly declare any constructors. However, it does implicitly declare a constructor that is called by classes that include the mixin. Since this constructor is not const, this means that mixins can't ever be used by classes with const constructors.

One possible solution to this would be to have the implicit mixin constructor be const for mixins with no instance variables.

[lrhn]

I assume you mean "mixin application" when you say "mixin", and is not referring to the class that is used as mixin (which does have an implicit default constructor).

The implicit "forwarding" constructors on a mixin application are indeed all non-const. You can't make them const if the superclass constructor that they forward to isn't also const, but in that case, I would also very much want that to be done when possible for all mixin application of classes with no non-final instance fields.

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Added Duplicate label.
Marked as being merged into #9745.