add previous token field

pkg/front_end/lib/src/fasta/scanner/array_based_scanner.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.array_based_scanner;
 
 import 'error_token.dart' show ErrorToken;
 
 import 'keyword.dart' show Keyword;
 
@@ skipping 22 matching lines @@
    * Each BeginGroupToken has a pointer to the token where the group
    * ends. This field is set when scanning the end group token.
    */
   Link<BeginGroupToken> groupingStack = const Link<BeginGroupToken>();
 
   /**
    * 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) {
       tail.precedingComments = comments;
       comments = null;
       commentsTail = null;
     }
   }
 
   /**
    * Appends a fixed token whose kind and content is determined by [info].
@@ skipping 165 matching lines @@
   }
 
   void appendComment(start, bool asciiOnly) {
     if (!includeComments) return;
     Token newComment = createSubstringToken(COMMENT_INFO, start, asciiOnly);
     if (comments == null) {
       comments = newComment;
       commentsTail = comments;
     } else {
       commentsTail.next = newComment;
+      commentsTail.next.previousToken = commentsTail;
       commentsTail = commentsTail.next;
     }
   }
 
   void appendErrorToken(ErrorToken token) {
     hasErrors = true;
     appendToken(token);
   }
 
   void appendSubstringToken(PrecedenceInfo info, int start, bool asciiOnly,
@@ skipping 25 matching lines @@
    * something which cannot possibly be part of a type parameter/argument
    * list, like the '=' in the above example.
    */
   void discardOpenLt() {
     while (!groupingStack.isEmpty &&
         identical(groupingStack.head.kind, LT_TOKEN)) {
       groupingStack = groupingStack.tail;
     }
   }
 }