[extension types] Member conflicts ignore precluded members

[eernstg]

See dart-lang/sdk#53719 (comment) for some background.

This PR adds rules to specify that a precluded member does not contribute to a conflict. This is a small adjustment to the rules which were added in #3470.

For example:

extension type E1(int it) {
  void foo() {}
}

extension type E2(int it) {
  int get foo => 0;
}

extension type E3(int it) implements E1, E2 {
  set foo(_) {} // No setter/method conflict occurs here.
}

void main() {
  var e3 = E3(1);
  e3.foo = true; // OK, `E3` has this setter.
  e3.foo; // OK, `E3` has this getter.
  e3.foo(); // Error, `E3` does not have this method, and the getter doesn't return a function.
}

The rationale is as follows: If E3 had had implements E1 then E3 would have had the setter named foo= and it would not have had the method named foo, because it is precluded. With implements E2, E3 would have had the setter and the getter, and there is no conflict (and no error). Hence, we might as well let E3 have the setter and the getter when it has implements E1, E2, because the consistent choice is in any case that E3 has the getter and not the method, so there is no ambiguity in E3 about the meaning of foo.

[eernstg]

Hi @jakemac53, could you take a look at this one? It's a correction to the rules about precluding an "otherwise-inherited" member of an extension type that were introduced in #3470.

[jakemac53]

Sorry its been a busy week, LGTM :)

[eernstg]

NP, thanks! ;-)