Add exception about extension members in error about nullable receivers

[eernstg]

Consider the following example:

extension on int? {
  bool get isEvenOrNull {
    var self = this; // To get promotion.
    return self == null || self.isEven;
  }
}

void main() {
  int? iq = 2 > 1 ? null : 42;
  bool b = iq.isEvenOrNull; // OK.
}

As implemented, the analyzer and CFE do not report any errors for iq.isEvenOrNull. However, the NNBD feature specification says that

It is an error to call a method, setter, getter or operator on an expression whose type is potentially nullable and not dynamic, except for the methods, setters, getters, and operators on Object.

Arguably, this makes extension member invocations an error when the receiver is potentially nullable, even in the case where the given extension is applicable to a receiver whose type is nullable (as in the example).

I believe the implemented behavior is the desired behavior, and the specification should be adjusted.

This change is non-breaking because the implementations already accept this kind of invocation.