anchor.diff:    int get m1 => 0;
     void set m2(int x) {}
     int get mc => 0;
  +  void operator[]=(int index, int value) {}
   }
   
   extension E2 on C2 {
     void set m1(int x) {}
     int get m2 => 0;
     String get me => "";
  +  int operator[](int index) => 0;
  +
   
     void test2() {
       // Using `this.member` means using the `on` type.
  
positive.code:   // [cfe] unspecified
  
    c0.m1 += 0;
    // ^^^^^^
    // [analyzer] unspecified
    // [cfe] unspecified
  
    c0.m2 = 0;
    c0.m2;
    // ^^
    // [analyzer] unspecified
    // [cfe] unspecified
    c0.m2 += 0;
    // ^^^^^^^
    // [analyzer] unspecified
    // [cfe] unspecified
    E0(c0).m2;
  }
  
  // Conflicting extensions.
  
negative.code: class C0 {
    int get m1 => 0;
    void set m2(int x) {}
    int operator[](int index) => 0;
  }
  
  extension E0 on C0 {
    void set m1(int x) {}
    int get m2 => 0;
    void operator[]=(int index, int value) {}
  }
  
  void test0() {
    C0 c0 = C0();
    c0.m1;
    c0.m1 = 0;
    // ^^
    // [analyzer] STATIC_WARNING.ASSIGNMENT_TO_FINAL_NO_SETTER
    // [cfe] unspecified
    E0(c0).m1 = 0;
    E0(c0).m1;
  