Language/Classes/definition_t23 is incorrect

[kmillikin]

The spec in the repository (https://github.com/dart-lang/sdk/blob/master/docs/language/dartLangSpec.tex) currently has:

It is a compile-time error if the superclass of a class $C$ is specified as a superinterface of $C$.

The named test checks that we can compile classes like:

@A() abstract class I3<T> = A1 with B1 implements A1;
abstract class J3<T> = A1 with B1 implements A1;

@A() abstract class I4 = A1 with B1 implements A1;
abstract class J4 = A1 with B1 implements A1;
@B() class X4 = A1 with B1 implements A1;
class Y4 = A1 with B1 implements A1;

which should all be a compile-time error. See dart-lang/sdk#31624.

[sgrekhov]

Fixed