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

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

 // Copyright (c) 2012, 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.abstract_scanner;
 
 import 'dart:collection' show ListMixin;
 
 import 'dart:typed_data' show Uint16List, Uint32List;
 
@@ skipping 847 matching lines @@
   }
 
   /**
    * Append the given token to the [tail] of the current stream of tokens.
    */
   void appendToken(Token token) {
     tail.next = token;
     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;
     }
   }
 
   void _appendToCommentStream(Token newComment) {
     if (comments == null) {
       comments = newComment;
       commentsTail = comments;
@@ skipping 403 matching lines @@
       switchToUint32(newLength);
     }
   }
 
   void switchToUint32(int newLength) {
     final newArray = new Uint32List(newLength);
     newArray.setRange(0, arrayLength, array);
     array = newArray;
   }
 }