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

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

 // Copyright (c) 2011, 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.token;
 
 import '../../scanner/token.dart' as analyzer;
 
 import 'keyword.dart' show Keyword;
 
@@ skipping 445 matching lines @@
 
   @override
   Token copyWithoutComments() =>
       new StringToken._(info, valueOrLazySubstring, charOffset);
 
   @override
   String value() => lexeme;
 }
 
 class CommentToken extends StringToken implements analyzer.CommentToken {
+  @override
+  analyzer.TokenWithComment parent;
+
   /**
    * Creates a lazy comment token. If [canonicalize] is true, the string
    * is canonicalized before the token is created.
    */
   CommentToken.fromSubstring(
       PrecedenceInfo info, String data, int start, int end, int charOffset,
       {bool canonicalize: false})
       : super.fromSubstring(info, data, start, end, charOffset,
             canonicalize: canonicalize);
 
@@ skipping 12 matching lines @@
       : super.fromUtf8Bytes(info, data, start, end, asciiOnly, charOffset);
 
   CommentToken._(PrecedenceInfo info, valueOrLazySubstring, int charOffset)
       : super._(info, valueOrLazySubstring, charOffset);
 
   @override
   CommentToken copy() =>
       new CommentToken._(info, valueOrLazySubstring, charOffset);
 
   @override
-  analyzer.TokenWithComment get parent {
-    Token token = next;
-    while (token is CommentToken) {
-      token = token.next;
+  void remove() {
+    if (previous != null) {
+      previous.setNextWithoutSettingPrevious(next);
+      next?.previous = previous;
+    } else {
+      assert(parent.precedingComments == this);
+      parent.precedingComments = next as CommentToken;
     }
-    return token;
-  }
-
-  @override
-  void set parent(analyzer.TokenWithComment ignored) {
-    throw 'unsupported operation';
-  }
-
-  @override
-  void remove() {
-    // TODO: implement remove
-    throw 'not implemented yet';
   }
 }
 
 class DartDocToken extends CommentToken
     implements analyzer.DocumentationCommentToken {
   /**
    * The references embedded within the documentation comment.
    * This list will be empty unless this is a documentation comment that has
    * references embedded within it.
    */
@@ skipping 118 matching lines @@
       identical(value, "<=") ||
       identical(value, "<") ||
       identical(value, "&") ||
       identical(value, "^") ||
       identical(value, "|");
 }
 
 bool isTernaryOperator(String value) => identical(value, "[]=");
 
 bool isMinusOperator(String value) => identical(value, "-");