anchor.diff:  
   // Will be instantiated with U=A and V=B.
   class Test3<U extends A, V extends B> {
  -  final U x = const A(); //# 11: ok
  +  final U x = const A(); //# 11: compile-time error
     final U x = const B(); //# 12: compile-time error
     final V x = const A(); //# 13: compile-time error
     final V x = const C(); //# 14: compile-time error
  
positive.code: 
  // Will be instantiated with U=A and V=B.
  class Test4<U extends A, V extends A> {
    final U x = const A(); //# 16: ok
    final U x = const B(); //# 17: compile-time error
    final V x = const A(); //# 18: compile-time error
    final V x = const C(); //# 19: compile-time error
  
negative.code: 
  // Will be instantiated with U=dynamic and V=dynamic.
  class Test5<U extends A, V extends B> {
    final U x = const A(); //# 21: compile-time error
    final U x = const B(); //# 22: compile-time error
    final V x = const A(); //# 23: compile-time error
    final V x = const C(); //# 24: compile-time error
    final V x = const C.d(); //# 25: compile-time error
    const Test5();
  