anchor.diff:  }
   
   class A2 extends C2 //
  -    with M2 //# 05: compile-time error
  +    with M2 //# 08: compile-time error
   {
     // M2.bar does a super.foo, so C2.foo must satisfy the super-interface of M2.
     // It doesn't, even if the super-call would succeed against C1.foo.
  
positive.code: 
  main() {
    A1().bar(); //# 04: continued
    A2().bar(); //# 05: continued
  }
negative.code:   }
  }
  
  mixin M11 on UnaryNum {
    num bar() {
      return 0.0;
    }
    num foo(num x) {
      return super.foo(42.0);
    }
  }
  
  // The super-invoked method must be non-abstract.
  class A1 extends UnaryNum //
      with M1 //# 04: compile-time error
      with M11 //# 05: compile-time error
      with M1, M11 //# 06: compile-time error
      with M11, M1 //# 07: compile-time error
  {
    // M1.bar does super.foo and UnaryNum has no implementation.
    num foo(num x) => x;
  