flatten token hierarchy

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 BeginToken, TokenType, TokenWithComment;
+import '../../scanner/token.dart' show BeginToken, SimpleToken, TokenType;
 
 import '../fasta_codes.dart'
     show
         Code,
         codeAsciiControlCharacter,
         codeEncoding,
         codeExpectedHexDigit,
         codeMissingExponent,
         codeNonAsciiIdentifier,
         codeNonAsciiWhitespace,
@@ skipping 40 matching lines @@
 
     default:
       return new NonAsciiIdentifierToken(character, charOffset);
   }
 }
 
 /// Common superclass for all error tokens.
 ///
 /// It's considered an implementation error to access [lexeme] of an
 /// [ErrorToken].
-abstract class ErrorToken extends TokenWithComment {
+abstract class ErrorToken extends SimpleToken {
   ErrorToken(int offset) : super(TokenType.BAD_INPUT, offset, null);
 
   /// This is a token that wraps around an error message. Return 1
   /// instead of the size of the length of the error message.
   @override
   int get length => 1;
 
   String get lexeme => throw assertionMessage;
 
   String get assertionMessage;
@@ skipping 136 matching lines @@
   UnmatchedToken(BeginToken begin)
       : this.begin = begin,
         super(begin.charOffset);
 
   String toString() => "UnmatchedToken(${begin.lexeme})";
 
   String get assertionMessage => "'$begin' isn't closed.";
 
   Code get errorCode => codeUnmatchedToken;
 }