Add CommentToken.parent and implement CommentToken.remove().

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

Index: pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
diff --git a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
index 38a7e614f938ade38d508bbc8d560969a7623ed7..60bfaaf6fe5e9d914e84688404c1e5ea25899169 100644
--- a/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
+++ b/pkg/front_end/lib/src/fasta/scanner/abstract_scanner.dart
@@ -865,6 +865,9 @@ abstract class AbstractScanner implements Scanner {
     tail.next.previousToken = tail;
     tail = tail.next;
     if (comments != null) {
+      for (CommentToken c = comments; c != null; c = c.next) {
+        c.parent = tail;
+      }
       tail.precedingCommentTokens = comments;
       comments = null;
       commentsTail = null;