HINT.UNREACHABLE_SWITCH_CASE is required for the analyzer

[sgrekhov]

Fix LanguageFeatures/Extension-types/exhaustiveness_variable_A04_t01.dart.

See https://github.com/dart-lang/sdk/blob/main/pkg/test_runner/lib/src/command_output.dart

  /// The set of hints which must be expected in a test. Any hint not specified
  /// here which is reported by the analyzer does not need to be expected, and
  /// never causes a test to fail.
  static const Set<String> _specifiedHints = {
    'unreachable_switch_case',
  };