Fix co19 exhaustiveness tests

[sgrekhov]

Update

LanguageFeatures/Patterns/Exhaustiveness/exhaustiveness_sealed_A02_t04.dart
LanguageFeatures/Patterns/Exhaustiveness/exhaustiveness_sealed_A02_t05.dart
LanguageFeatures/Patterns/Exhaustiveness/exhaustiveness_sealed_A02_t06.dart
LanguageFeatures/Patterns/Exhaustiveness/exhaustiveness_sealed_A02_t07.dart

according to the dart-lang/sdk#53392 (comment)