move comment tokens into preceedingComments field

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

Index: pkg/front_end/lib/src/fasta/scanner/token.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/token.dart b/pkg/front_end/lib/src/fasta/scanner/token.dart
index 6cab122a2689477af7f47ffe1bba68287b98e105..0c29f1c6ad0baadb7181d53042fa36e4634611a8 100644
--- a/pkg/front_end/lib/src/fasta/scanner/token.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/token.dart
@@ -29,6 +29,14 @@ abstract class Token {
   Token next;
 
   /**
+   * Return the first comment in the list of comments that precede this token,
+   * or `null` if there are no comments preceding this token. Additional
+   * comments can be reached by following the token stream using [next] until
+   * `null` is returned.
+   */
+  Token precedingComments;
+
+  /**
    * The precedence info for this token. [info] determines the kind and the
    * precedence level of this token.
    *