align fasta.Token and analyzer.Token

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

 // Copyright (c) 2017, 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 licenset hat can be found in the LICENSE file.
 
 library dart_scanner.error_token;
 
 import '../../scanner/token.dart' show TokenType;
 
 import '../fasta_codes.dart'
     show
@@ skipping 55 matching lines @@
 ///
 /// It's considered an implementation error to access [lexeme] of an
 /// [ErrorToken].
 abstract class ErrorToken extends Token {
   ErrorToken(int charOffset) : super(charOffset);
 
   TokenType get type => TokenType.BAD_INPUT;
 
   String get lexeme => throw assertionMessage;
 
-  String get stringValue => null;
-
   bool get isIdentifier => false;
 
   String get assertionMessage;
 
   FastaCode get errorCode;
 
   int get character => null;
 
   String get start => null;
 
@@ skipping 129 matching lines @@
   UnmatchedToken(BeginGroupToken begin)
       : this.begin = begin,
         super(begin.charOffset);
 
   String toString() => "UnmatchedToken(${begin.lexeme})";
 
   String get assertionMessage => "'$begin' isn't closed.";
 
   FastaCode get errorCode => codeUnmatchedToken;
 }