I believe this issue and #33644 have the same cause: fasta doesn't allow mixin applications to have const constructors.

/cc @askeksa-google

I believe that the title may give rise to some confusion. Maybe it could be 'It is an error for a class to have a constant constructor if its superclass is a mixin application S with M where M has a field'?

The "interesting" case where an error arises even though everything seems to be OK is the following:

class S { const S.foo(); }
class M { final x = 42; }
class A extends S with M { const A.foo(): super.foo(); }

In this situation we could have written the mixin application as a regular class (with an explicit declaration of the forwarding constant constructor), and everything would just work:

class S { const S.foo(); }
class SwithM extends S { final x = 42;  const SwithM.foo(): super.foo(); }
class A extends SwithM { const A.foo(): super.foo(); }

So even in this case where "it should just work", we get an error because the mechanism that generates forwarding constructors just won't generate a constant one when there is a field.

The justification for making this an error is that a final field with a constant initializing expression is a pretty useless thing. ;-)