move comment tokens into preceedingComments field

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

Index: pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
index ca004ed5d38b549637bf81bdaf5a922e04efd87d..84ae1554d696d7e82606a5f1518642b0dface19e 100644
--- a/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/utf8_bytes_scanner.dart
@@ -198,11 +198,12 @@ class Utf8BytesScanner extends ArrayBasedScanner {
   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;