fasta tokens implement analyzer token API

pkg/analyzer/lib/src/fasta/token_utils.dart

 // Copyright (c) 2016, 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.analyzer.token_utils;
 
 import 'package:front_end/src/fasta/scanner/error_token.dart' show ErrorToken;
 
 import 'package:front_end/src/fasta/scanner/keyword.dart' show Keyword;
 
@@ skipping 136 matching lines @@
   /// fasta may contain synthetic tokens from error recovery that are not mapped
   /// to the analyzer token stream.  We use [_beginTokenStack] and
   /// [_endTokenStack] to create the appropriate links for non-synthetic tokens
   /// in the way analyzer expects.
   void _matchGroups(Token token, analyzer.Token translatedToken) {
     if (identical(_endTokenStack.last, token)) {
       _beginTokenStack.last.endToken = translatedToken;
       _beginTokenStack.removeLast();
       _endTokenStack.removeLast();
     }
-    // Synthetic end tokens use the same offset as the begin token.
+    // Synthetic end tokens have a length of zero.
     if (translatedToken is analyzer.BeginToken &&
         token is BeginGroupToken &&
         token.endGroup != null &&
         token.endGroup.charOffset != token.charOffset) {
       _beginTokenStack.add(translatedToken);
       _endTokenStack.add(token.endGroup);
     }
   }
 }
 
@@ skipping 571 matching lines @@
     case PERIOD_PERIOD_PERIOD_TOKEN:
       return TokenType.PERIOD_PERIOD_PERIOD;
     case GENERIC_METHOD_TYPE_LIST_TOKEN:
       return TokenType.GENERIC_METHOD_TYPE_LIST;
     case GENERIC_METHOD_TYPE_ASSIGN_TOKEN:
       return TokenType.GENERIC_METHOD_TYPE_ASSIGN;
     default:
       return internalError("Unhandled token ${token.info}");
   }
 }