anchor.diff:  
   main() {
     A1().bar(); //# 04: continued
  -  A2().bar(); //# 05: continued
  +  A1().bar(); //# 05: continued
  +  A1().bar(); //# 06: continued
  +  A1().bar(); //# 07: continued
  +  A2().bar(); //# 08: continued
   }
positive.code:   }
  }
  
  // The super-invoked method must be non-abstract.
  class A1 extends UnaryNum //
      with M1 //# 04: compile-time error
  {
    // M1.bar does super.foo and UnaryNum has no implementation.
    num foo(num x) => x;
  
negative.code: }
  
  class A2 extends C2 //
      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.
  