VM complains about extends/implements clauses that the user didn't write.

[lrhn]

Example:

class C {}
mixin M on C, Object {}
main(){}

This program gives the error message:

mix2.dart:1: Error: 'Object' can't be used in both 'extends' and 'implements' clauses.
Try removing one of the occurrences.

There is no implements clause in the program. There is an implicit super-interface for the mixin, class M$Super implements C, Object {}, but that has no extends clause.

The spec required error for writing the same type as both a super-class and an implements class, or twice as an implements class, should not trigger unless the user wrote the clause.

No other platform gives this error.