move comment tokens into preceedingComments field

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 180 matching lines @@
     if (stringOffsetSlackOffset == byteOffset) {
       return byteOffset - utf8Slack - 1;
     } else {
       return byteOffset - utf8Slack;
     }
   }
 
   Token firstToken() => tokens.next;
   Token previousToken() => tail;
 
-  void appendSubstringToken(PrecedenceInfo info, int start, bool asciiOnly,
+  @override
+  StringToken createSubstringToken(
+      PrecedenceInfo info, int start, bool asciiOnly,
       [int extraOffset = 0]) {
-    tail.next = new StringToken.fromUtf8Bytes(
+    return new StringToken.fromUtf8Bytes(
         info, bytes, start, byteOffset + extraOffset, asciiOnly, tokenStart);
-    tail = tail.next;
   }
 
   bool atEndOfFile() => byteOffset >= bytes.length - 1;
 }