anchor.diff:  
     var actual8 = c2
         ?.. m1()?.m()
  -      .. m2()?.m();                   /// static type warning
  +      .. m2()?.m();
     Expect.equals(expected2, actual8);
     Expect.equals(2, c2.m1().counter);
     Expect.equals(1, c2.m2().counter);
  
positive.code: 
    var actual6 = c2
      ?.. m1()?.m()
      .. m2()?.m();                     /// static type warning
    Expect.isNull(actual6);
  
    c2 = new C();
  
negative.code: 
    var actual4 = c1
        ?.. m1()?.m()                   /// static type warning
        .. m2()?.m();
    Expect.equals(expected, actual4);
    Expect.equals(2, c1.m2().counter);
  
  