cleanup fasta token classes

pkg/front_end/lib/src/fasta/scanner.dart

 // Copyright (c) 2016, the Dart project authors. Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE.md file.
 
 library fasta.scanner;
 
 import 'dart:convert' show UNICODE_REPLACEMENT_CHARACTER_RUNE, UTF8;
 
 import '../scanner/token.dart' show Token;
 
 import 'scanner/string_scanner.dart' show StringScanner;
 
 import 'scanner/utf8_bytes_scanner.dart' show Utf8BytesScanner;
 
 import 'scanner/recover.dart' show defaultRecoveryStrategy;
 
 export 'scanner/token.dart'
     show
-        BeginGroupToken,
         StringToken,
-        SymbolToken,
         isBinaryOperator,
         isMinusOperator,
         isTernaryOperator,
         isUnaryOperator,
         isUserDefinableOperator;
 
 export 'scanner/error_token.dart'
     show ErrorToken, buildUnexpectedCharacterToken;
 
 export 'scanner/token_constants.dart' show EOF_TOKEN;
@@ skipping 57 matching lines @@
 ScannerResult _tokenizeAndRecover(Scanner scanner, Recover recover,
     {List<int> bytes, String source}) {
   Token tokens = scanner.tokenize();
   if (scanner.hasErrors) {
     if (bytes == null) bytes = UTF8.encode(source);
     recover ??= defaultRecoveryStrategy;
     tokens = recover(bytes, tokens, scanner.lineStarts);
   }
   return new ScannerResult(tokens, scanner.lineStarts, scanner.hasErrors);
 }