fasta tokens implement analyzer token API

pkg/analyzer/lib/src/dart/ast/token.dart

 // Copyright (c) 2014, 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 analyzer.src.dart.ast.token;
 
-import 'package:front_end/src/scanner/token.dart';
-
 export 'package:front_end/src/scanner/token.dart'
     show
         BeginToken,
         BeginTokenWithComment,
         CommentToken,
         DocumentationCommentToken,
         KeywordToken,
         KeywordTokenWithComment,
         SimpleToken,
         StringToken,
         StringTokenWithComment,
+        SyntheticKeywordToken,
+        SyntheticStringToken,
         TokenClass,
         TokenWithComment;
-
-/**
- * A token whose value is independent of it's type.
- */
-class SyntheticStringToken extends StringToken {
-  /**
-   * Initialize a newly created token to represent a token of the given [type]
-   * with the given [value] at the given [offset].
-   */
-  SyntheticStringToken(TokenType type, String value, int offset)
-      : super(type, value, offset);
-
-  @override
-  bool get isSynthetic => true;
-}