cleanup duplicate fasta scanner methods

pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart

 // Copyright (c) 2013, 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 file.
 
 library fasta.scanner.utf8_bytes_scanner;
 
 import 'dart:convert' show UNICODE_BOM_CHARACTER_RUNE, UTF8;
 
 import '../scanner.dart' show unicodeReplacementCharacter;
 
@@ skipping 177 matching lines @@
   int stringOffsetSlackOffset = -1;
 
   int get stringOffset {
     if (stringOffsetSlackOffset == byteOffset) {
       return byteOffset - utf8Slack - 1;
     } else {
       return byteOffset - utf8Slack;
     }
   }
 
-  Token firstToken() => tokens.next;
-  Token previousToken() => tail;
-
   @override
   StringToken createSubstringToken(
       PrecedenceInfo info, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new StringToken.fromUtf8Bytes(
         info, bytes, start, byteOffset + extraOffset, asciiOnly, tokenStart);
   }
 
   bool atEndOfFile() => byteOffset >= bytes.length - 1;
 }