[NNBD] default List constructor

[sgrekhov]

According to the NNBD spec

It is an error to call the default List constructor with a length argument and a type argument which is potentially non-nullable.

There are two conditions conbined with AND. It looks strange for me. Please consider the following code.

class A {}

class C<X extends A?> {
  X x;
  C(this.x);

  test() {
    List l1 = new List<X>();     // type X is potentially non-nullable but no length argument specified. No error?
    List l2 = new List<X>(0);    // type X is potentially non-nullable, argument specified. Is this an error then?
    List l3 = <X>[];    // what about this?
    var l4 = <X>[x];    // and this?
}

main() {
  A? a = new A();
  C<A?> c = new C<A?>(a);
  c.test();
}

[eernstg]

Cf. discussions at the meeting yesterday: The problem is that the length argument implies that the new list should have that many elements, but a potentially non-nullable type argument does not allow for any values to be used (can't use null, and no other values can be used as a natural default value for an unknown type like X extends Object).

Also, List<X>(0) is an error even though it is safe, because the error check does not bother to discover that this particular argument value is safe; List<X>(1) must be an error because we can't obtain a value of type X out of the blue.

List<X>() is OK following the spec text on the error.

Finally, list literals are irrelevant, because this error is only concerned with explicit invocations of that particular List constructor.