cleanup duplicate fasta scanner methods

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

 // Copyright (c) 2011, 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 dart2js.scanner.string_scanner;
 
 import 'array_based_scanner.dart' show ArrayBasedScanner;
 
 import 'precedence.dart' show PrecedenceInfo;
 
@@ skipping 23 matching lines @@
 
   int advance() => string.codeUnitAt(++scanOffset);
   int peek() => string.codeUnitAt(scanOffset + 1);
 
   int get stringOffset => scanOffset;
 
   int currentAsUnicode(int next) => next;
 
   void handleUnicode(int startScanOffset) {}
 
-  Token firstToken() => tokens.next;
-  Token previousToken() => tail;
-
   @override
   StringToken createSubstringToken(
       PrecedenceInfo info, int start, bool asciiOnly,
       [int extraOffset = 0]) {
     return new StringToken.fromSubstring(
         info, string, start, scanOffset + extraOffset, tokenStart,
         canonicalize: true);
   }
 
   bool atEndOfFile() => scanOffset >= string.length - 1;
 }