Fixes #3028. Add tests for mixin application constant constructor

[sgrekhov]

No description provided.

[eernstg]

LGTM.

I was just wondering about one thing: The tests starting with ...A03_t06... seem to differ by having different kinds of parameters (optional named, required named). I couldn't see an obvious reason why this would help testing anything which is concerned with constant constructors?

If there is a need to change anything then we can do it in a new PR, I'll land this one now.

[sgrekhov]

I was just wondering about one thing: The tests starting with ...A03_t06... seem to differ by having different kinds of parameters (optional named, required named). I couldn't see an obvious reason why this would help testing anything which is concerned with constant constructors?

There are 3 statements in the specification, about constructor with no optional parameters, with optional positional parameters and with named parameters. ...A03... tests checking mixin application constructor with named parameters.
...t06: optional named parameter, mixin has a static variable, mixin application of the form class MA = A with M;
...t07: required named parameter, mixin has a static variable, mixin application of the form class MA = A with M;
...t08: optional named parameter, mixin has a static variable, mixin application of the form class MA extends A with M...
...t09: required named parameter, mixin has a static variable, mixin application of the form class MA extends A with M...
and 4 more test but with the instance variable in a mixin.

I believe it makes sense to check all combinations. What if mixin application constructor looses its constantness, say, in case of required named parameter only? See, for example, dart-lang/sdk#59796.