[private named parameters] Allow non-reserved word public names.

To use a private named parameter, the corresponding public name must be
a valid identifier. One way it could be invalid is by being a reserved
word, as in:

```dart
class C({final String _if}); // Error.
```

However, other special identifiers that are not actually reserved words
are allowed:

```dart
class C({final String _async}); // OK.
```

This fixes that.

Change-Id: I6755a613910ae97e7db9b6a7f70e1cf0dd379b17
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465740
Auto-Submit: Bob Nystrom <[email protected]>
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: munificent

pkg/_fe_analyzer_shared/lib/src/scanner/token_impl.dart

@@ -10,13 +10,14 @@ import 'characters.dart';
 
 import 'token.dart'
     show
+        CommentToken,
         DocumentationCommentToken,
+        Keyword,
+        KeywordStyle,
+        LanguageVersionToken,
         SimpleToken,
-        TokenType,
-        CommentToken,
         StringToken,
-        LanguageVersionToken,
-        Keyword;
+        TokenType;
 
 import 'token_constants.dart' show IDENTIFIER_TOKEN;
 
@@ -375,9 +376,13 @@ String? correspondingPublicName(String identifier) {
   // digit.
   if (isDigit(firstCharacter)) return null;
 
-  // The resulting name must not be a reserved word.
+  // The resulting name must not be a reserved word (but other kinds of special
+  // identifiers are allowed).
   String publicName = identifier.substring(1);
-  if (Keyword.keywords.containsKey(publicName)) return null;
+  if (Keyword.keywords[publicName] case var keyword?
+      when keyword.keywordStyle == KeywordStyle.reserved) {
+    return null;
+  }
 
   return publicName;
 }