[Wildcard Variables] Resolution of wildcard import directives

[pq]

As per dart-lang/language#3799, import prefixes named _ are non-binding but will provide access to the non-private extensions in that library.

// a.dart
extension ExtendedString on String {
  bool get isBlank => trim().isEmpty;
}

// b.dart
import 'a.dart' as _; // OK.

main() {
  print(''.isBlank); // Prints `true`.
}

See: https://github.com/dart-lang/language/blob/main/working/wildcards/feature-specification.md#imports