Report compile-time errors on cyclic hierarchies.

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;
 
 import 'precedence.dart'
     show
         AS_INFO,
         BAD_INPUT_INFO,
         EOF_INFO,
         IDENTIFIER_INFO,
         IS_INFO,
         KEYWORD_INFO,
-        MULTI_LINE_COMMENT_INFO,
-        SINGLE_LINE_COMMENT_INFO,
         PrecedenceInfo;
 
 import 'token_constants.dart' show IDENTIFIER_TOKEN;
 
 import 'string_canonicalizer.dart';
 
 /**
  * A token that doubles as a linked list.
  */
 abstract class Token implements analyzer.TokenWithComment {
@@ skipping 597 matching lines @@
       identical(value, "<=") ||
       identical(value, "<") ||
       identical(value, "&") ||
       identical(value, "^") ||
       identical(value, "|");
 }
 
 bool isTernaryOperator(String value) => identical(value, "[]=");
 
 bool isMinusOperator(String value) => identical(value, "-");