[Augmentations] No error is reported by the analyzer if augmented is used in switch in augmentation

[sgrekhov]

The following expected error is not reported by the analyzer.

// main.dart
import augment 'aug_lib.dart';

const augmented = "Constant augmented, shouldn't be used";
void set topLevelSetter(String value) {}

// aug_lib.dart
augment library 'main.dart';

augment void set topLevelSetter(String value) {
  switch ("") {
    case augmented:
//       ^^^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
    default:
  }
  var x = switch("") {
    augmented => 1,
//  ^^^^^^^^^
// [analyzer] unspecified
// [cfe] unspecified
    _ => 0
  };
}

See, for example, https://github.com/dart-lang/co19/blob/master/LanguageFeatures/Augmentation-libraries/augmented_expression_A02_t24.dart

[keertip]

@scheglov

[srawlins]

Ping on a P1 issue; feel free to re-categorize this as an "enhancement."