Add test for part with multiple owners

[sgrekhov]

Add test for part with multiple owners. Like

// multiple_owners_part.dart
part of multiple_owners_lib;
final definedInPart = "Lily was here";

// multiple_owners_owner_t01.dart
library multiple_owners_lib;
part 'multiple_owners_part.dart';

var definedInPart = "";

main() {
}

// multiple_owners_owner_t02.dart
library multiple_owners_lib;
part 'multiple_owners_part.dart';

main() {
  print(definedInPart);
}

Waiting for dart-lang/sdk#54661 and dart-lang/sdk#54836 to figure our correct error expectations

[sgrekhov]

Parts with import removed "< dottedIdentifier >" from a "< partHeader >", retaining only "< uri >". It's also a syntax error to have multiple "part of" directives. Therefore with "Parts with imports" feature it is not possible to have a part file with multiple owners. Closing this issue.