LanguageFeatures/Extension-types/exhaustiveness_map_A02_t02 needs updating for CL 345822

[stereotype441]

CL 345822 (which addresses dart-lang/language#3534) modifies flow analysis so that recognizes when a single pattern is guaranteed to match because its erased type is either the same as, or a supertype of, the erased type of the matched value. This partially addresses a situation where a switch statement is recognized by the exhaustiveness checker as exhaustive, but not by flow analysis.

Such a situation occurs in LanguageFeatures/Extension-types/exhaustiveness_map_A02_t02:

extension type ET1<K, V>(Map<K, V> _) {}

String test(ET1<bool, bool> m) {
//     ^^^^
// [analyzer] unspecified
// [cfe] unspecified
  switch (m) {
    ...
    case Map():
      return "other";
  }
}

Currently the above code expects an error, because flow analysis cannot tell that the pattern Map() is guaranteed to match the extension type ET1<bool, bool>. After the CL lands, flow analysis will recognize that the Map() pattern is guarnateed to match, so there will be no error.

[sgrekhov]

Fixed by #2503